Absence of yolk sac hematopoiesis from mice with a targeted disruption of the scl gene

Hematopoietic commitment during embryogenesis

Hematopoiesis develops initially as discrete blood islands in the extraembryonic yolk sac of the embryo. These blood islands consist of clusters of primitive erythrocytes surrounded by developing angioblasts that ultimately form the yolk sac vasculature. The close developmental association of these early hematopoietic and endothelial cells has led to the hypothesis that they develop from a common precursor, a cell known as the hemangioblast. Using a developmental model system based on the in vitro differentiation capacity of embyronic stem (ES) cells, we have identified a precursor with the capacity to generate endothelial as well as primitive and definitive hematopoietic progeny. The developmental potential of this precursor population suggests that it represents the in vitro equivalent of the hemangioblast.

Erythroid gene expression is differentially regulated by erythropoietin, haemin and delta-aminolaevulinic acid in UT-7 cells

Erythropoietin (Epo) is essential for the later stages of erythropoiesis, acting to promote cell survival and proliferation, but its role in differentiation remains to be defined. The UT-7 cell line exhibits both erythroid and megakaryocytic characteristics and can be induced to differentiate along the erythroid pathway by Epo or the megakaryocytic pathway by phorbol myristic acetate. We have compared the effects of Epo and the chemical inducers, delta-aminolaevulinic acid (delta-ALA) and haemin on the differentiation capacity of UT-7 cells. Epo alone promoted relatively early events in erythroid maturation, without significant changes in haemoglobin production or morphology. GATA-2 and c-myb were down-regulated by Epo, and GATA-2 was further down-modulated by the inducers. Conversely, SCL expression was up-regulated by Epo and further increased by haemin and delta-ALA. Epo caused an increase in the proportion of cells expressing cell surface glycophorin A (GPA) and up-regulated beta- and gamma-globin by several fold. Both haemin and delta-ALA caused a de novo increase in alpha-globin expression as well as enhancing Epo-induced beta-globin expression, leading to a marked increase in haemoglobin production. These results suggest that haemoglobin production in UT-7 cells is limited by a deficiency of erythroid-specific aminolaevulinic acid synthase (ALAS-E) activity or globin synthesis as a consequence of their immaturity as a multipotential cell line.

The leukemic oncogene tal-2 is expressed in the developing mouse brain

tal-1 (T-cell acute leukemia-1; also known as SCL) and tal-2 genes belong to a family of basic helix-loop-helix transcription factors and were originally isolated from the breakpoints of chromosomal translocations in human T-cell leukemia cell lines. tal-1 is expressed not only in hematopoietic cells but also in several endothelial structures and the central nervous system during development. On the other hand, the detailed function and the sites of expression of tal-2 have remained obscure. We cloned the tal-2 cDNA from a mouse embryonic cDNA library and examined its expression pattern in the mouse, comparing with that of tal-1. In situ analyses revealed that tal-2 transcripts are detected at embryonic day 12.5 in the following regions; 1) the diencephalon-the zona limitans intrathalamica and the pretectum, 2) the mesencephalon-the tectum, and the anterior and posterior tegmentum, 3) the metencephalon-the isthmus and the anterior pons. In the diencephalon and the mesencephalon, the expression sites of tal-2 gene were similar to those of tal-1, and its expression was stronger than that of tal-1. In the metencephalon, tal-2 expression was observed in the anterior pons, whereas tal-1 transcripts were detected in the entire pons, and showed stronger expression than tal-2. The tal-2 messages were barely detectable in the brain at birth. These results suggest that tal-1 and tal-2 are involved in the development of specific areas of the central nervous system.

Prolonged expression of c-fos suppresses cell cycle entry of dormant hematopoietic stem cells

The proto-oncogene c-fos was transiently upregulated in primitive hematopoietic stem (Lin-Sca-1(+)) cells stimulated with stem cell factor, interleukin-3 (IL-3), and IL-6. To investigate a role of the c-fos in hematopoietic stem cells, we used bone marrow (BM) cells from transgenic mice carrying the c-fos gene under the control of the interferon-alpha/beta-inducible Mx-promoter (Mx-c-fos), and fetal liver cells from c-fos-deficient mice. Prolonged expression of the c-fos in Lin-Sca-1(+) BM cells inhibited factor-dependent colony formation and hematopoiesis on a stromal cell layer by keeping them at G0/G1 phase of the cell cycle. These Lin-Sca-1(+) BM cells on a stromal layer entered into the cell cycle whenever exogenous c-fos was downregulated. However, ectopic c-fos did not perturb colony formation by Lin-Sca-1(+) BM cells after they entered the cell cycle. Furthermore, endogenous c-fos is not essential to cell cycle progression of hematopoietic stem cells because the factor-dependent and the stroma-dependent hematopoiesis by Lin-Sca-1(+) fetal liver cells from c-fos-deficient mice was not impaired. These results suggest that the c-fos induced in primitive hematopoietic stem cells negatively controls cell cycle progression and maintains them in a dormant state.

Prolonged Expression of c-fos Suppresses Cell Cycle Entry of Dormant Hematopoietic Stem Cells

The proto-oncogene c-fos was transiently upregulated in primitive hematopoietic stem (Lin−Sca-1+) cells stimulated with stem cell factor, interleukin-3 (IL-3), and IL-6. To investigate a role of the c-fos in hematopoietic stem cells, we used bone marrow (BM) cells from transgenic mice carrying the c-fos gene under the control of the interferon-/β–inducible Mx-promoter (Mx–c-fos), and fetal liver cells from c-fos–deficient mice. Prolonged expression of the c-fos in Lin−Sca-1+ BM cells inhibited factor-dependent colony formation and hematopoiesis on a stromal cell layer by keeping them at G0/G1 phase of the cell cycle. These Lin−Sca-1+ BM cells on a stromal layer entered into the cell cycle whenever exogenous c-fos was downregulated. However, ectopic c-fos did not perturb colony formation by Lin−Sca-1+ BM cells after they entered the cell cycle. Furthermore, endogenous c-fos is not essential to cell cycle progression of hematopoietic stem cells because the factor-dependent and the stroma-dependent hematopoiesis by Lin−Sca-1+ fetal liver cells from c-fos–deficient mice was not impaired. These results suggest that the c-fos induced in primitive hematopoietic stem cells negatively controls cell cycle progression and maintains them in a dormant state.

Self-renewal and differentiation of a basic fibroblast growth factor-dependent multipotent hematopoietic cell line derived from embryonic stem cells

Despite the accumulation of informat on on the origin of hematopoietic stem cells, it is still unclear how these cells are generated in ontogeny. Isolation of cell lines equivalent to early embryonic hematopoietic progenitor cells can be helpful. A multipotent hematopoietic progenitor cell line, A-6, was isolated from H-1 embryonic stem (ES) cells. The self-renewal of A-6 cells was supported by basic-fibroblast growth factor (b-FGF) and their differentiation into definitive erythroid cells, granulocytes and macrophages was induced after co-culture with ST-2 stromal cells. A-6 cells were positive for the surface markers of hematopoietic stem cell, c-kit, CD31, CD34, Flt3/Flk2, PgP-1, and HSA, but were negative for that of the differentiated cells. Reverse transcription-polymerase chain reaction analysis showed that A-6 cells produced mRNA from SCL/tal-1 and GATA-2 genes. Among various cytokines examined, on y stem cell factor (SCF) and Flt3/Flk2 ligand (FL) supported the proliferation of A-6 cells instead of b-FGF. The FL, as well as b-FGF, supported the self-renewal of A-6 cells, whereas SCF induced differentiation into myeloid cells. A-6 cells will be useful for the characterization of hematopoietic progenitor cells derived from ES cells and provide a model system to realize the control mechanisms between self-renewal and different ation of hematopoietic stem cells.

Physical interaction of the bHLH LYL1 protein and NF-kappaB1 p105

The LYL1 gene was first identified upon the molecular characterization of the t(7;9)(q35;p13) translocation associated with some human T-cell acute leukemias (T-ALLs). In adult tissues, LYL1 expression is restricted to hematopoietic cells with the notable exclusion of the T cell lineage. LYL1 encodes a basic helix-loop-helix (bHLH) protein highly related to TAL-1, whose activation is also associated with a high proportion of human T-ALLs. A yeast two-hybrid system was used to identify proteins that specifically interact with LYL1 and might mediate its activities. We found that p105, the precursor of NF-kappaB1 p50, was the major LYL1-interacting protein in this system. The association between LYL1 and p105 was confirmed both in vitro and in vivo in mammalian cells. Biochemical studies indicated that the interaction was mediated by the bHLH motif of LYL1 and the ankyrin-like motifs of p105. Ectopic expression of LYL1 in a human T cell line caused a significant decrease in NF-kappaB-dependent transcription, associated with a reduced level of NF-kappaB1 proteins.

Retinoid X receptors are essential for early mouse development and placentogenesis

Embryos carrying null mutations of both retinoid X receptors alpha and beta (RXRalpha-/-/RXRbeta-/- mutants) were generated. These mutant embryos die between 9.5 and 10.5 days of gestation and display a wide range of abnormalities. The cause of the lethality appears to be the lack of formation of the labyrinthine zone of the chorioallantoic placenta. In a thorough analysis of mutant conceptuses, we establish that RXRs, through heterodimerization with retinoic acid receptors, are essential for postimplantation embryonic development before placentogenesis. RXRs are also essential for the formation of the chorioallantoic placenta, most probably through RXR/peroxisomal proliferator-activated receptor-gamma heterodimers. Interestingly, as a RXR ligand appears dispensable, placentogenesis must be controlled by a yet unknown hormonal ligand(s) activating the heterodimerization partner(s) of RXRs.

The hematopoietic transcription factor SCL binds the p44 subunit of TFIIH

SCL is a basic domain helix-loop-helix (bHLH) oncoprotein that is involved in T-cell acute lymphoblastic leukemia as well as in normal hematopoiesis. Although it is believed that SCL functions as a tissue-specific transcription factor, no molecular mechanism has thus far been identified for this putative function. In this report, we show that SCL interacts with p44, a subunit of the basal transcription factor TFIIH. The minimal region of SCL that interacts with p44 was mapped to a 101-amino acid sequence that includes, but is not limited to, the bHLH region; the SCL-binding site of p44 is located in the carboxyl-terminal half of p44. This interaction was confirmed by glutathione S-transferase fusion protein pull-down assays and a co-immunoprecipitation assay. As analyzed with a yeast two-hybrid system, p44 interacts specifically with SCL, but not with the other class A or B bHLH proteins tested. E2A did not compete with p44 for SCL binding, as demonstrated by an in vitro binding assay. These findings document a previously unsuspected interaction between SCL and a subunit of the basal transcription factor TFIIH, suggesting a potential means by which SCL might modulate transcription.

Perspective: chromosomal translocations can affect genes controlling gene expression and differentiation--why are these functions targeted?

Chromosomal translocations are important aetiological factors in many human cancers. These aberrant chromosomes cause enforced expression of oncogenes located near the breakpoints or results in tumour-specific fusion proteins. Among the characteristics which influence the tumourigenic effect, it is observed that the genes at translocation junctions are often transcription factors and often normally involved in developmental processes. Furthermore, protein-protein interactions are key elements in the mechanism by which the translocation gene products exert their pathogenic effects. In this review some of these salient features are discussed and generalizations are suggested which may be applicable to the influence of chromosomal translocations on acute forms of cancer.

Failure of ventral closure and axial rotation in embryos lacking the proprotein convertase Furin

We have examined the role of Furin in postimplantation-stage mouse embryos by analyzing both the expression pattern of fur mRNA and the developmental consequences of a loss-of-function mutation at the fur locus. At early stages (day 7.5), fur mRNA is abundant in extraembryonic endoderm and mesoderm, anterior visceral endoderm, and in precardiac mesoderm. 1 day later fur is expressed throughout the heart tube and in the lateral plate mesoderm, notochordal plate and definitive gut endoderm. Embryos lacking Furin die between days 10.5 and 11.5, presumably due to hemodynamic insufficiency associated with severe ventral closure defects and the failure of the heart tube to fuse and undergo looping morphogenesis. Morphogenesis of the yolk sac vasculature is also abnormal, although blood islands and endothelial precursors form. Analysis of cardiac and endodermal marker genes shows that while both myocardial precursors and definitive endoderm cells are specified, their numbers and migratory properties are compromised. Notably, mutant embryos fail to undergo axial rotation, even though Nodal and eHand, two molecular markers of left-right asymmetry, are appropriately expressed. Overall, the present data identify Furin as an important activator of signals responsible for ventral closure and embryonic turning.

TAL1 and LIM-only proteins synergistically induce retinaldehyde dehydrogenase 2 expression in T-cell acute lymphoblastic leukemia by acting as cofactors for GATA3

Previously, we have shown that TAL1 and the LIM-only protein gene (LMO) are regularly coactivated in T-cell acute lymphoblastic leukemia (T-ALL). This observation is likely to relate to the findings that TAL1 and LMO are highly synergistic in T-cell tumorigenesis in double-transgenic mice. To understand the molecular mechanisms of functional synergy between TAL1 and LMO in tumorigenesis and transcriptional regulation, we tried to identify downstream target genes regulated by TAL1 and LMO by a subtractive PCR method. One of the isolated genes, that for retinaldehyde dehydrogenase 2 (RALDH2), was regularly expressed in most of the T-ALL cell lines that coexpressed TAL1 and LMO. Exogenously transfected TAL1 and LMO, but not either alone, induced RALDH2 expression in a T-ALL cell line, HPB-ALL, not expressing endogeneous TAL1 or LMO. The RALDH2 transcripts in T-ALL were, however, mostly initiated within the second intron. Promoter analysis revealed that a GATA site in a cryptic promoter in the second intron was essential and sufficient for the TAL1- and LMO-dependent transcriptional activation, and GATA3 binds to this site. In addition, forced expression of GATA3 potentiated the induction of RALDH2 by TAL1 and LMO, and these three factors formed a complex in vivo. Furthermore, a TAL1 mutant not binding to DNA also activated the transcription of RALDH2 in the presence of LMO and GATA3. Collectively, we have identified the RALDH2 gene as a first example of direct transcriptional target genes regulated by TAL1 and LMO in T-ALL. In this case, TAL1 and LMO act as cofactors for GATA3 to activate the transcription of RALDH2.

Reduction of DNA binding activity of the GATA-1 transcription factor in the apoptotic process induced by overexpression of PU.1 in murine erythroleukemia cells

Previously we have shown that overexpression of PU.1, an Ets family transcription factor, in murine erythroleukemia (MEL) cells results in apoptotic cell death in the presence of the differentiation-inducing reagent dimethyl sulfoxide (DMSO). In this study, we examined the dynamics of GATA-1 and NF-E2 hematopoietic transcription factors during the induction of apoptosis, because GATA-1 has been shown to be implicated in survival of erythroid cells. Formation of the GATA-1-DNA complex as judged by EMSA was markedly reduced when apoptosis was induced, although subcellular localization of the GATA-1 protein and expression levels of the GATA-1 mRNA and protein were not changed during the apoptotic process. Complex formation was not reduced when apoptosis was avoided by adding 30% serum in culture medium and when mutant PU.1 proteins with the deletion of the DNA-binding (Ets) or transactivation domain were expressed. Complex formation in nuclear extracts of parental MEL cells was reduced when they were mixed with those of apoptotic cells, suggesting that apoptotic cells may contain a factor(s) preventing GATA-1 from binding to DNA. In contrast to GATA-1, formation of the NF-E2-DNA complex was not changed during the process of apoptosis, although the expression level of the NF-E2 p45 gene was reduced in the process. These results suggest that reduction of the DNA-binding activity of GATA-1 may partly account for PU.1-mediated apoptosis in MEL cells.

SCL binds the human homologue of DRG in vivo

A suspected oncoprotein, human development regulated GTP-binding protein (DRG) has never been identified though homologues were found in mouse, Xenopus, Drosophila, yeast and Halobacteria. During a search for SCL binding partners using the yeast 2-hybrid system, we isolated two independent cDNA clones (clone L51 and clone V3) of the human DRG homologue from human fetal liver and human thymus cDNA libraries. Only one amino acid difference was found between human and mouse DRG proteins. Although a human DRG has been previously deposited in the SWISS-PROT Database, we believe that we have cloned the bona fide human DRG based on the highly conserved primary amino acid structure between our cloned human homologue and the mouse DRG.

Intraaortic hemopoietic cells are derived from endothelial cells during ontogeny

We have investigated the developmental relationship of the hemopoietic and endothelial lineages in the floor of the chicken aorta, a site of hemopoietic progenitor emergence in the embryo proper. We show that, prior to the onset of hemopoiesis, the aortic endothelium uniformly expresses the endothelium-specific membrane receptor VEGF-R2. The onset of hemopoiesis can be determined by detecting the common leukocyte antigen CD45. VEGF-R2 and CD45 are expressed in complementary fashion, namely the hemopoietic cluster-bearing floor of the aorta is CD45(+)/VEGF-R2(−), while the rest of the aortic endothelium is CD45(−)/VEGF-R2(+). To determine if the hemopoietic clusters are derived from endothelial cells, we tagged the E2 endothelial tree from the inside with low-density lipoproteins (LDL) coupled to DiI. 24 hours later, hemopoietic clusters were labelled by LDL. Since no CD45(+) cells were inserted among endothelial cells at the time of vascular labelling, hemopoietic clusters must be concluded to derive from precursors with an endothelial phenotype.

Transcriptional regulation of the stem cell leukemia gene by PU.1 and Elf-1

The SCL gene, also known as tal-1, encodes a basic helix-loop-helix transcription factor that is pivotal for the normal development of all hematopoietic lineages. SCL is expressed in committed erythroid, mast, and megakaryocytic cells as well as in hematopoietic stem cells. Nothing is known about the regulation of SCL transcription in mast cells, and in other lineages GATA-1 is the only tissue-specific transcription factor recognized to regulate the SCL gene. We have therefore analyzed the molecular mechanisms underlying SCL expression in mast cells. In this paper, we demonstrate that SCL promoter 1a was regulated by GATA-1 together with Sp1 and Sp3 in a manner similar to the situation in erythroid cells. However, SCL promoter 1b was strongly active in mast cells, in marked contrast to the situation in erythroid cells. Full activity of promoter 1b was dependent on ETS and Sp1/3 motifs. Transcription factors PU.1, Elf-1, Sp1, and Sp3 were all present in mast cell extracts, bound to promoter 1b and transactivated promoter 1b reporter constructs. These data provide the first evidence that the SCL gene is a direct target for PU.1, Elf-1, and Sp3.

EphA4 (Sek1) receptor tyrosine kinase is required for the development of the corticospinal tract

Members of the Eph family of tyrosine kinase receptors have been implicated in the regulation of developmental processes and, in particular, axon guidance in the developing nervous system. The function of the EphA4 (Sek1) receptor was explored through creation of a null mutant mouse. Mice with a null mutation in the EphA4 gene are viable and fertile but have a gross motor dysfunction, which is evidenced by a loss of coordination of limb movement and a resultant hopping, kangaroo-like gait. Consistent with the observed phenotype, anatomical studies and anterograde tracing experiments reveal major disruptions of the corticospinal tract within the medulla and spinal cord in the null mutant animals. These results demonstrate a critical role for EphA4 in establishing the corticospinal projection.

Morphogenesis of the first blood vessels

The initial phase of vessel formation is the establishment of nascent endothelial tubes from mesodermal precursor cells. Development of the vascular epithelium is examined using the transcription factor TAL1 as a marker of endothelial precursor cells (angioblasts), and a functional assay based on intact, whole-mounted quail embryos. Experimental studies examining the role(s) of integrins and vascular endothelial growth factor (VEGF) establish that integrin-mediated cell adhesion is necessary for normal endothelial tube formation and that stimulation of embryonic endothelial cells with exogenous VEGF results in a massive "fusion" of vessels and the obliteration of normally avascular zones. The second phase of vessel morphogenesis is assembly of the vessel wall. To understand the process by which mesenchyme gives rise to vascular smooth muscle, a novel monoclonal antibody, 1E12, that recognizes smooth muscle precursor cells was used. Additionally, development of the vessel wall was examined using the expression fo extracellular matrix proteins as markers. Comparison of labeling patterns of 1E12 and the extracellular matrix molecules fibulin-1 and fibrillin-2 indicate vessel wall heterogeneity at the earliest stages of development; thus smooth muscle cell diversity is manifested during the differentiation and assembly of the vessel wall. From these studies it is postulated that the extracellular matrix composition of the vessel wall may prove to be the best marker of smooth muscle diversity. The data are discussed in the context of recent work by others, especially provocative new studies suggesting an endothelial origin for vascular smooth muscle cells. Also discussed is recent work that provides clues to the mechanism of vascular smooth muscle induction and recruitment. Based on these findings, vascular smooth muscle cells can be thought of as existing along a continuum of phenotypes. This spectrum varies from mainly matrix-producing cells to primarily contractile cells; thus no one cell type typifies vascular smooth muscle. This view of the smooth muscle cell is considered in terms of a contrasting opinion that views smooth muscle cell as existing in either a synthetic or proliferative state.

A transcription factor party during blood cell differentiation

Recent studies have shown that hematopoietic transcription factors can engage in multiple protein-protein interactions. Accumulating evidence indicates that specific complexes define differentiation lineages and differentiation stages. It is proposed that these complexes acquire new functions during blood cell differentiation through successive changes in composition - much as discussion topics of groups at a cocktail party take new directions as new people join and others leave.

Characterization of hematopoietic progenitor cells that express the transcription factor SCL, using a lacZ "knock-in" strategy

Gene targeting experiments have demonstrated that the transcription factor SCL is essential for primitive and definitive hematopoiesis in the mouse. To study the functional properties of hematopoietic cells expressing SCL, we have generated mutant mice (SCLlacZ/w) in which the Escherichia coli lacZ reporter gene has been "knocked in" to the SCL locus, thereby linking beta-galactosidase expression to transcription from the SCL promoter. Bone marrow cells from heterozygous SCLlacZ/w mice were sorted into fractions expressing high, intermediate and low levels of beta-galactosidase (designated lacZhigh, lacZint, and lacZneg). Cells that were lacZhigh or lacZint were enriched for day 12 spleen colony-forming units and myeloid and erythroid colony-forming cells (CFCs). These fractions included >99% of the erythroid and >90% of the myeloid CFCs. Culture of sorted bone marrow populations on stromal cells secreting interleukin-7 or in fetal thymic organ cultures showed that B and T lymphoid progenitors were also present in the lacZhigh and lacZint fractions. These data provide a functional correlation between SCL expression and colony-forming ability in immature hematopoietic cells. Our data also suggested that expression of SCL was transient and confined to hematopoietic stem and/or progenitor cells, because the differentiated progeny of most lineages (except the erythroid) were beta-galactosidase-negative.

epicardin: A novel basic helix-loop-helix transcription factor gene expressed in epicardium, branchial arch myoblasts, and mesenchyme of developing lung, gut, kidney, and gonads

We report the cloning, chromosomal localization, and analysis of the expression pattern of epicardin, a member of the basic helix-loop-helix (bHLH) family of transcription factors. Within its bHLH domain, the human and murine epicardin genes were most similar to paraxis, a bHLH gene important for segmentation of embryonic paraxial mesoderm. In situ hybridization studies revealed strong epicardin expression in murine embryos at 9.5 days postcoitum (dpc) in a region of the septum transversum at the base of the heart known as the proepicardial organ. This mesenchymal structure extends villous projections from which epicardial precursor cells emerge and migrate out over the surface of the myocardium. Strong expression was seen in individual migratory cells and clusters at 9.5 dpc and in a continuous epicardial cell layer in more mature hearts. Also from 9.5 dpc, epicardin transcripts were seen in endocardial cushions of the atrioventricular canal and outflow tract, in skeletal myoblasts within branchial arches and in condensing mesenchyme of gut, kidney, urinary tract, gonads, spleen, and lung. Northern analysis showed that expression persisted in mature visceral organs and heart, but was transient in skeletal muscle. The central role played by bHLH factors in pathways for tissue determination in the embryo suggests a function for epicardin in specification of select mesodermal cell populations associated with heart, cranial skeletal muscle, gut, and urogenital system.

Chromatin immunoselection defines a TAL-1 target gene

Despite the major functions of the basic helix-loop-helix transcription factor TAL-1 in hematopoiesis and T-cell leukemogenesis, no TAL-1 target gene has been identified. Using immunoprecipitation of genomic fragments bound to TAL-1 in the chromatin of murine erythro-leukemia (MEL) cells, we found that 10% of the immunoselected fragments contained a CAGATG or a CAGGTG E-box, followed by a GATA site. We studied one of these fragments containing two E-boxes, CAGATG and CAGGTC, followed by a GATA motif, and showed that TAL-1 binds to the CAGGTG E-box with an affinity modulated by the CAGATG or the GATA site, and that the CAGGTG-GATA motif exhibits positive transcriptional activity in MEL but not in HeLa cells. This immunoselected sequence is located within an intron of a new gene co-expressed with TAL-1 in endothelial and erythroid cells, but not expressed in fibroblasts or adult liver where no TAL-1 mRNA was detected. Finally, in vitro differentiation of embryonic stem cells towards the erythro/megakaryocytic pathways showed that the TAL-1 target gene expression followed TAL-1 and GATA-1 expression. These results establish that TAL-1 is likely to activate its target genes through a complex that binds an E-box-GATA motif and define the first gene regulated by TAL-1.

The oncogenic T cell LIM-protein Lmo2 forms part of a DNA-binding complex specifically in immature T cells

The LIM-only protein LMO2 is expressed aberrantly in acute T-cell leukaemias as a result of the chromosomal translocations t(11;14) (p13;q11) or t(7;11) (q35;p13). In a transgenic model of tumorigenesis by Lmo2, T-cell acute leukaemias arise after an asymptomatic phase in which an accumulation of immature CD4(-) CD8(-) double negative thymocytes occurs. Possible molecular mechanisms underlying these effects have been investigated in T cells from Lmo2 transgenic mice. Isolation of DNA-binding sites by CASTing and band shift assays demonstrates the presence of an oligomeric complex involving Lmo2 which can bind to a bipartite DNA motif comprising two E-box sequences approximately 10 bp apart, which is distinct from that found in erythroid cells. This complex occurs in T-cell tumours and it is restricted to the immature CD4(- )CD8(-) thymocyte subset in asymptomatic transgenic mice. Thus, ectopic expression of Lmo2 by transgenesis, or by chromosomal translocations in humans, may result in the aberrant protein interactions causing abnormal regulation of gene expression, resulting in a blockage of T-cell differentiation and providing precursor cells for overt tumour formation.

Bcl-3 Expression and Nuclear Translocation Are Induced by Granulocyte-Macrophage Colony-Stimulating Factor and Erythropoietin in Proliferating Human Erythroid Precursors

Bcl-3 is a proto-oncogene involved in the chromosomal translocation t(14;19) found in some patients with chronic lymphocytic leukemia. It shares structural similarities with and is a member of the IκB family of proteins. In this report, involvement of Bcl-3in hematopoietic growth factor-stimulated erythroid proliferation and differentiation was examined. In TF-1 cells, an erythroleukemia cell line, granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Epo) greatly enhanced Bcl-3 expression at both the protein and mRNA levels in association with stimulation of proliferation. Bcl-3 protein was also highly expressed in early burst-forming unit-erythroid (BFU-E)–derived erythroid precursors (day 7) and decreased during maturation (days 10 and 14), suggesting that Bcl-3 is involved in normal erythroid proliferation. In these hematopoietic cells, Bcl-3 was hyperphosphorylated. GM-CSF and Epo modulated the subcellular localization of Bcl-3. Upon stimulation of TF-1 cells with GM-CSF or Epo, the nuclear translocation ofBcl-3 was dramatically enhanced. Overexpression of Bcl-3 in TF-1 cells by transient transfection along with the NF-κB factors p50 or p52 resulted in significant induction of an human immunodeficiency virus–type 1 (HIV-1) κB-TATA-luceriferase reporter plasmid, demonstrating that Bcl-3 has a positive role in transactivation of κB-containing genes in erythroid cells. Stimulation with GM-CSF enhanced c-myb mRNA expression in these cells. Bcl-3 in nuclear extracts of TF-1 cells bound to a κB enhancer in the c-mybpromoter together with NF-κB2/p52 and this binding activity was enhanced by GM-CSF stimulation. Furthermore, cotransfection of Bcl-3 with p52 or p50 in TF-1 cells resulted in significant activation of ac-myb κB-TATA-luceriferase reporter plasmid. These findings suggest that Bcl-3 may participate in the transcriptional regulation of certain κB-containing genes involved in hematopoiesis, includingc-myb.

© 1998 by The American Society of Hematology.

Bcl-3 Expression and Nuclear Translocation Are Induced by Granulocyte-Macrophage Colony-Stimulating Factor and Erythropoietin in Proliferating Human Erythroid Precursors

Bcl-3 is a proto-oncogene involved in the chromosomal translocation t(14;19) found in some patients with chronic lymphocytic leukemia. It shares structural similarities with and is a member of the IκB family of proteins. In this report, involvement of Bcl-3in hematopoietic growth factor-stimulated erythroid proliferation and differentiation was examined. In TF-1 cells, an erythroleukemia cell line, granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Epo) greatly enhanced Bcl-3 expression at both the protein and mRNA levels in association with stimulation of proliferation. Bcl-3 protein was also highly expressed in early burst-forming unit-erythroid (BFU-E)–derived erythroid precursors (day 7) and decreased during maturation (days 10 and 14), suggesting that Bcl-3 is involved in normal erythroid proliferation. In these hematopoietic cells, Bcl-3 was hyperphosphorylated. GM-CSF and Epo modulated the subcellular localization of Bcl-3. Upon stimulation of TF-1 cells with GM-CSF or Epo, the nuclear translocation ofBcl-3 was dramatically enhanced. Overexpression of Bcl-3 in TF-1 cells by transient transfection along with the NF-κB factors p50 or p52 resulted in significant induction of an human immunodeficiency virus–type 1 (HIV-1) κB-TATA-luceriferase reporter plasmid, demonstrating that Bcl-3 has a positive role in transactivation of κB-containing genes in erythroid cells. Stimulation with GM-CSF enhanced c-myb mRNA expression in these cells. Bcl-3 in nuclear extracts of TF-1 cells bound to a κB enhancer in the c-mybpromoter together with NF-κB2/p52 and this binding activity was enhanced by GM-CSF stimulation. Furthermore, cotransfection of Bcl-3 with p52 or p50 in TF-1 cells resulted in significant activation of ac-myb κB-TATA-luceriferase reporter plasmid. These findings suggest that Bcl-3 may participate in the transcriptional regulation of certain κB-containing genes involved in hematopoiesis, includingc-myb.

© 1998 by The American Society of Hematology.

Transcription factor SCL is required for c-kit expression and c-Kit function in hemopoietic cells

In normal hemopoietic cells that are dependent on specific growth factors for cell survival, the expression of the basic helix-loop-helix transcription factor SCL/Tal1 correlates with that of c-Kit, the receptor for Steel factor (SF) or stem cell factor. To address the possibility that SCL may function upstream of c-kit, we sought to modulate endogenous SCL function in the CD34(+) hemopoietic cell line TF-1, which requires SF, granulocyte/macrophage colony-stimulating factor, or interleukin 3 for survival. Ectopic expression of an antisense SCL cDNA (as-SCL) or a dominant negative SCL (dn-SCL) in these cells impaired SCL DNA binding activity, and prevented the suppression of apoptosis by SF only, indicating that SCL is required for c-Kit-dependent cell survival. Consistent with the lack of response to SF, the level of c-kit mRNA and c-Kit protein was significantly and specifically reduced in as-SCL- or dn-SCL- expressing cells. c-kit mRNA, c-kit promoter activity, and the response to SF were rescued by SCL overexpression in the antisense or dn-SCL transfectants. Furthermore, ectopic c-kit expression in as-SCL transfectants is sufficient to restore cell survival in response to SF. Finally, enforced SCL in the pro-B cell line Ba/F3, which is both SCL and c-kit negative is sufficient to induce c-Kit and SF responsiveness. Together, these results indicate that c-kit, a gene that is essential for the survival of primitive hemopoietic cells, is a downstream target of the transcription factor SCL.

The hemangioblast--an elusive cell captured in culture

The close temporal and spatial association between blood and endothelial cell development during embryogenesis was first documented almost 100 years ago. In recent years, gene expression studies have further strengthened this link. Now, using cultures of mouse embryonic stem cells, a common progenitor cell that gives rise to both blood cells and vascular endothelial cells, has been identified. The existence of the hemangioblast has been proved and experiments addressing its unique properties can begin.

Overexpression of HOX11 Leads to the Immortalization of Embryonic Precursors With Both Primitive and Definitive Hematopoietic Potential

Primitive and definitive erythropoiesis represent distinct hematopoietic programs that differ with respect to stage of development, transcriptional control, and growth regulation. Although these differences have been recognized for some time, the relationship of the two erythroid lineages to each other is not well established. We have used a model system based on the hematopoietic development of embryonic stem (ES) cells in culture to investigate the origins of the earliest hematopoietic populations. Using ES cells transduced with a retrovirus that overexpresses the HOX11 gene, we have established factor-dependent hematopoietic cell lines that represent novel stages of embryonic hematopoiesis. Analysis of three of these cell lines indicates that they differ with respect to cytokine responsiveness, cell surface markers, and developmental potential. Two of the cell lines, EBHX1 and EBHX11, display the unique capacity to generate both primitive and definitive erythroid progeny as defined by morphology and expression of βH1 and βmajor globin. The third line, EBHX14, has definitive erythroid and myeloid potential, but is unable to generate cells of the primitive erythroid lineage. Analysis of the cytokine responsiveness of the two lines with primitive erythroid potential has indicated that exposure to leukemia inhibitory factor (LIF) results in the upregulation of βH1 and a change in cellular morphology to that of primitive erythrocytes. These findings are the first demonstration of a clonal cell line with primitive and definitive hematopoietic potential and support the interpretation that these lineages may arise from a common precursor in embryonic life. In addition, they suggest that LIF could play a role in the regulation of primitive erythropoiesis.

© 1998 by The American Society of Hematology.

Overexpression of HOX11 Leads to the Immortalization of Embryonic Precursors With Both Primitive and Definitive Hematopoietic Potential

Primitive and definitive erythropoiesis represent distinct hematopoietic programs that differ with respect to stage of development, transcriptional control, and growth regulation. Although these differences have been recognized for some time, the relationship of the two erythroid lineages to each other is not well established. We have used a model system based on the hematopoietic development of embryonic stem (ES) cells in culture to investigate the origins of the earliest hematopoietic populations. Using ES cells transduced with a retrovirus that overexpresses the HOX11 gene, we have established factor-dependent hematopoietic cell lines that represent novel stages of embryonic hematopoiesis. Analysis of three of these cell lines indicates that they differ with respect to cytokine responsiveness, cell surface markers, and developmental potential. Two of the cell lines, EBHX1 and EBHX11, display the unique capacity to generate both primitive and definitive erythroid progeny as defined by morphology and expression of βH1 and βmajor globin. The third line, EBHX14, has definitive erythroid and myeloid potential, but is unable to generate cells of the primitive erythroid lineage. Analysis of the cytokine responsiveness of the two lines with primitive erythroid potential has indicated that exposure to leukemia inhibitory factor (LIF) results in the upregulation of βH1 and a change in cellular morphology to that of primitive erythrocytes. These findings are the first demonstration of a clonal cell line with primitive and definitive hematopoietic potential and support the interpretation that these lineages may arise from a common precursor in embryonic life. In addition, they suggest that LIF could play a role in the regulation of primitive erythropoiesis.

© 1998 by The American Society of Hematology.

SCL specifies hematopoietic mesoderm in Xenopus embryos

Targeted gene disruption experiments in the mouse have demonstrated an absolute requirement for several transcription factors for the development of hematopoietic progenitors during embryogenesis. Disruption of the basic helix-loop-helix gene SCL (stem cell leukemia) causes a block early in the hematopoietic program with defects in all hematopoietic lineages. To understand how SCL participates in the organogenesis of blood, we have isolated cDNAs encoding Xenopus SCL and characterized the function of SCL during embryogenesis. We demonstrate that SCL is expressed in ventral mesoderm early in embryogenesis. SCL expression is induced by BMP-4, and a dominant negative BMP-4 receptor inhibits SCL expression in the ventral region of the embryo. Expression of SCL in either bFGF-treated animal pole explants or dorsal marginal zone explants leads to the expression of globin protein. Furthermore, over-expression of SCL does not alter normal dorsal-ventral patterning in the embryo, indicating that SCL acts to specify mesoderm to a hematopoietic fate after inductive and patterning events have occurred. We propose that SCL is both necessary and sufficient to specify hematopoietic mesoderm, and that it has a similar role in specifying hematopoietic cell fate as MyoD has in specifying muscle cell fate.

The SCL gene specifies haemangioblast development from early mesoderm

The SCL gene encodes a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of all haematopoietic lineages. SCL is also expressed in endothelial cells, but its function is not essential for specification of endothelial progenitors and the role of SCL in endothelial development is obscure. We isolated the zebrafish SCL homologue and show that it was co-expressed in early mesoderm with markers of haematopoietic, endothelial and pronephric progenitors. Ectopic expression of SCL mRNA in zebrafish embryos resulted in overproduction of common haematopoietic and endothelial precursors, perturbation of vasculogenesis and concomitant loss of pronephric duct and somitic tissue. Notochord and neural tube formation were unaffected. These results provide the first evidence that SCL specifies formation of haemangioblasts, the proposed common precursor of blood and endothelial lineages. Our data also underline the striking similarities between the role of SCL in haematopoiesis/vasculogenesis and the function of other bHLH proteins in muscle and neural development.

Multiple proteins binding to a GATA-E box-GATA motif regulate the erythroid Krüppel-like factor (EKLF) gene

Erythroid Krüppel-like factor (EKLF) is a zinc finger transcription factor required for beta-globin gene expression and is implicated as one of the key factors necessary for the fetal to adult switch in globin gene expression. In an effort to identify factors involved in the expression of this important erythroid-specific regulatory protein, we have isolated the mouse EKLF gene and systematically analyzed the promoter region. Initially, a reporter construct with 1150 base pairs of the EKLF 5'-region was introduced into transgenic mice and shown to direct erythroid-specific expression. We continued the expression studies in erythroid cells and have identified a sequence element consisting of two GATA sites flanking an E box motif. The three sites act in concert to elevate the transcriptional activity of the EKLF promoter. Each site is essential for EKLF expression indicating that the three binding sites do not work additively, but rather function as a unit. We further show that GATA-1 binds to the two GATA sites and present evidence for binding of another factor from erythroid cell nuclear extracts to the E box motif. These results are consistent with the formation of a quaternary complex composed of an E box dimer and two GATA-1 proteins binding at a combined GATA-E box-GATA activator element in the distal EKLF promoter.

The making of an erythroid cell. Molecular control of hematopoiesis

The number of circulating red cells is regulated by the daily balance between two processes: the destruction of the old red cells in the liver and the generation of new cells in the bone marrow. The process during which hematopoietic stem cells generate new red cells is called erythropoiesis. This manuscript will describe the molecular mechanisms involved in the process of erythroid differentiation as we understand them today. In particular it will review how erythroid specific growth factor-receptor interactions activate specific transcription factors to turn on the expression of the genes responsible for the establishment of the erythroid phenotype.

Enhanced Megakaryocyte and Erythroid Development From Normal Human CD34+ Cells: Consequence of Enforced Expression of SCL

The product of the SCL gene is a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of hematopoietic stem cells in both the embryo and the adult. However, once the stem cell compartment is established, the function of SCL in subsequent differentiation and commitment events within normal hematopoietic cells remains undefined. The aim of the current study was to investigate this role using purified normal human hematopoietic CD34+ cells. An SCL retrovirus was used to transduce CD34+ cells isolated from human bone marrow, peripheral blood, and umbilical cord blood. Enforced expression of SCL increased by a median of twofold the number of erythroid colonies, with an increase in both colony size and the rate of hemoglobinization. Unexpectedly, enforced expression in CD34+ cells also significantly increased the number of megakaryocyte colonies, but with no impact on the size of colonies. There was no consistent effect on the number nor size of granulocyte-macrophage (GM) colonies. The proliferative effect of enforced SCL expression on erythroid cells was attributed to a shortened cell cycle time; the self-renewal capacity of erythroid or GM progenitors was unchanged, as was survival of cells within colonies. These results demonstrate a role for SCL in determining erythroid and megakaryocyte differentiation from normal human hematopoietic CD34+ cells.

Enhanced Megakaryocyte and Erythroid Development From Normal Human CD34+ Cells: Consequence of Enforced Expression of SCL

The product of the SCL gene is a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of hematopoietic stem cells in both the embryo and the adult. However, once the stem cell compartment is established, the function of SCL in subsequent differentiation and commitment events within normal hematopoietic cells remains undefined. The aim of the current study was to investigate this role using purified normal human hematopoietic CD34+ cells. An SCL retrovirus was used to transduce CD34+ cells isolated from human bone marrow, peripheral blood, and umbilical cord blood. Enforced expression of SCL increased by a median of twofold the number of erythroid colonies, with an increase in both colony size and the rate of hemoglobinization. Unexpectedly, enforced expression in CD34+ cells also significantly increased the number of megakaryocyte colonies, but with no impact on the size of colonies. There was no consistent effect on the number nor size of granulocyte-macrophage (GM) colonies. The proliferative effect of enforced SCL expression on erythroid cells was attributed to a shortened cell cycle time; the self-renewal capacity of erythroid or GM progenitors was unchanged, as was survival of cells within colonies. These results demonstrate a role for SCL in determining erythroid and megakaryocyte differentiation from normal human hematopoietic CD34+ cells.

Development of the definitive hematopoietic hierarchy in the mouse

Recent research on the ontogeny of the hematopoietic system in mammals has shown that a simple textbook steady-state hematopoietic hierarchy can not be strictly applied to the hematopoietic cells found within the embryo. During embryonic development, hematopoietic cells originate, migrate and differentiate in a number of distinct anatomical sites such as the yolk sac AGM region and liver and thus represent various classes of cells within diverse microenvironments. In this manuscript we review both cellular and molecular aspects of developmental hematopoiesis and present our current views on the numerous complex mechanisms underlying the establishment of definitive hematopoiesis.

The T cell leukemia LIM protein Lmo2 is necessary for adult mouse hematopoiesis

The LIM-finger protein Lmo2, which is activated in T cell leukemias by chromosomal translocations, is required for yolk sac erythropoiesis. Because Lmo2 null mutant mice die at embryonic day 9-10, it prevents an assessment of a role in other stages of hematopoiesis. We have now studied the hematopoietic contribution of homozygous mutant Lmo2 -/- mouse embryonic stem cells and found that Lmo2 -/- cells do not contribute to any hematopoietic lineage in adult chimeric mice, but reintroduction of an Lmo2-expression vector rescues the ability of Lmo2 null embryonic stem cells to contribute to all lineages tested. This disruption of hematopoiesis probably occurs because interaction of Lmo2 protein with factors such as Tal1/Scl is precluded. Thus, Lmo2 is necessary for early stages of hematopoiesis, and the Lmo2 master gene encodes a protein that has a central and crucial role in the hematopoietic development.

Role of PU.1 in hematopoiesis

The ETS-family transcription factor PU.1 is expressed in hematopoietic tissues, with significant levels of expression in the monocytic and B lymphocytic lineages. PU.1 is identical to the Spi-1 proto-oncogene which is associated with the generation of spleen focus-forming virus-induced erythroleukemias. An extensive body of in vitro gene regulatory studies has implicated PU.1 as an important, versatile regulator of B lymphoid- and myeloid-specific genes. The first half of the review is designed to coalesce data generated from studies examining the two PU.1 "knockout" animals, which have prompted a reevaluation of the proposed function of PU.1 during hematopoiesis. During hematopoiesis, PU.1 is required for development along the lymphoid and myeloid lineages but needs to be downregulated during erythropoiesis. These unique functional characteristics of PU.1 will be exemplified by contrasting the function of PU.1 with other transcription factors required during fetal hematopoiesis. The second half of this review will reexamine the functional characteristics of PU.1 deduced from traditional biochemical and transactivation assays in light of recent experiments examining the functional behavior of PU.1 in an embryonic stem cell in vitro differentiation system. Working models of how PU.1 regulates promoter and enhancer regions in the B cell and myeloid lineage will be presented and discussed.

The pufferfish SLP-1 gene, a new member of the SCL/TAL-1 family of transcription factors

The SCL/TAL-1 gene encodes a basic helix-loop-helix (bHLH) transcription factor essential for the development of all hemopoietic lineages and also acts as a T-cell oncogene. Four related genes have been described in mammals (LYL-1, TAL-2, NSCL1, and NSCL2), all of which exhibit a high degree of sequence similarity to SCL/TAL-1 in the bHLH domain and two of which (LYL-1 and TAL-2) have also been implicated in the pathogenesis of T-cell acute lymphoblastic leukemia. In this study we describe the identification and characterization of a pufferfish gene termed SLP-1, which represents a new member of this gene family. The genomic structure and sequence of SLP-1 suggests that it forms a subfamily with SCL/TAL-1 and LYL-1 and is most closely related to SCL/TAL-1. However, unlike SCL/TAL-1, SLP-1 is widely expressed. Sequence analysis of a whole cosmid containing SLP-1 shows that SLP-1 is flanked upstream by a zinc finger gene and a fork-head-domain gene and downstream by a heme-oxygenase and a RING finger gene.

Unsuspected role for the T-cell leukemia protein SCL/tal-1 in vascular development

The transcription factor SCL/tal-1 is essential for blood cell development. Though it is also expressed in vascular endothelium, SCL has been considered dispensable for vessel formation. Through transgenic rescue of hematopoietic defects of SCL-/- embryos and analysis of chimeras generated with SCL-/- ES cells tagged with a transgene expressed in vascular endothelial cells, we show that SCL is essential for angiogenic remodeling of the yolk sac capillary network into complex vitelline vessels. These findings establish a role for SCL in embryonic angiogenesis and argue for critical functions in both embryonic blood and vascular cells, the descendents of the presumptive hemangioblast.

Genetics of erythropoiesis: induced mutations in mice and zebrafish

Production of red blood cells (erythropoiesis) in the vertebrate embryo is critical to its survival and subsequent development. As red cells are the first blood cells to appear in embryogenesis, their origin reflects commitment of mesoderm to an hematopoietic fate and provides an avenue by which to examine the development of the hematopoietic system, including the hematopoietic stem cell (HSC). We discuss the genetics of erythropoiesis as studied in two systems: the mouse and zebrafish (Danio rerio). In the mouse, targeted disruption has established several genes as essential at different stages of hematopoiesis or erythroid precursor cell maturation. In the zebrafish, numerous mutants displaying a wide range of phenotypes have been isolated, although the affected genes are unknown. In comparing mouse knockout and zebrafish mutant phenotypes, we propose a pathway for erythropoiesis that emphasizes the apparent similarity of the mutants and the complementary nature of investigation in the two species. We speculate that further genetic studies in mouse and zebrafish will identify the majority of essential genes and define a regulatory network for hematopoiesis in vertebrates.

TAL1/SCL is expressed in endothelial progenitor cells/angioblasts and defines a dorsal-to-ventral gradient of vasculogenesis

In this study we establish that TAL1/SCL, a member of the helix-loop-helix family of transcription factors, and an important regulator of the hematopoietic lineage in mice, is expressed in the endothelial lineage of avians. The earliest events of vascular development were examined using antibodies to TAL1/SCL, and the QH1 antibody, an established marker of quail endothelial cells. Analyses using double immunofluorescence confocal microscopy show that: (i) TAL1/SCL is expressed by both quail and chicken endothelial cells; (ii) TAL1/SCL expression precedes that of the QH1 epitope; and (iii) TAL1/SCL, but not QH1, expression defines a subpopulation of primordial cells within the splanchnic mesoderm. Collectively these data suggest that TAL1/SCL-positive/QH1-negative cells are angioblasts. Further, using TAL1/SCL expression as a marker of the endothelial lineage, we demonstrate that in addition to the previously described cranial-to-caudal gradient, there is a dorsal-to-ventral progression of vasculogenesis.

Stem cell transcription factors

The individual properties of hematopoietic stem cells, including self-renewal, maintenance of pluripotency, and asymmetric cell division, must depend at some level on the functions of specific transcription factors. Recently, valuable insights into stem cell transcription factor function have emerged from targeted gene disruption (knockout) studies in mice. Absence of transcription factors belonging to diverse protein families results in interference with expansion and differentiation of either the stem cell itself or of primitive multipotential progenitors. The findings from these and complementary experiments provide a framework for examining the transcription control of the earliest cellular events in hematopoiesis.

Developmental biology of hematopoiesis

Hematopoietic stem cells are at the top of a hierarchy that regulates the generation of a vast repertoire of blood cells during the lifetime of a vertebrate. Recent experiments, using a vast variety of embryonic systems, shed new light on the origin of stem cells and the genes that function to regulate and maintain hematopoietic differentiation programs. Two distinct populations of stem cells develop--derived initially from transient, extraembryonic source and later from a stable, intraembryonic source; it is possible that both are generated from a pro-HSC able to respond differentially to local inductions. The initial blood cells develop from ventral mesoderm. The blood-forming region develops as a result of signaling from specific, secreted, embryonic growth factors, including the bone morphogenetic proteins. Stem cells give rise to progenitors that are restricted progressively in their ability to contribute to specific lineages. This process is regulated by transcription factors, whose functions are confirmed through genetic analyses. The identification of highly conserved, embryonic signaling pathways and transcription regulatory genes illustrates the enormous utility of analyzing embryonic hematopoiesis in frog, chick, fish, and mouse systems to further our understanding of human stem cells.

Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a rare, congenital, hypoplastic anemia that usually presents in early infancy. Congenital anomalies, particularly of the head and upper limbs, are present in about a quarter of reported patients. The disease is characterized by a moderate-to-severe macrocytic anemia, occasional neutropenia or thrombocytosis, a normocellular bone marrow with erythroid hypoplasia, and an increased risk of developing leukemia. The pathogenesis is unknown. The majority of patients respond to prednisone, and often erythropoiesis can be maintained with low doses of the drug. Both remissions and increased resistance to steroid treatment can occur. Nonresponders usually are transfusion dependent, although responses to high dose steroid, androgen, and interleukin-3 have been observed. Bone marrow transplantation can be curative.