Pluripotent, cytokine-dependent, hematopoietic stem cells are immortalized by constitutive Notch1 signaling

Progenitor cells of the biliary epithelial cell lineage

Stem-like cells have been identified in liver that are able to differentiate in vivo and in culture to biliary epithelial cells (BEC), hepatocytes and oval cells. The growth factors/cytokines and signal pathways required for the differentiation processes are beginning to be evaluated. There is increasing evidence to suggest that these stem-like cells may originate from both the bone marrow population or from a precursor remnant from liver embryogenesis, as they share many of the same markers (CD34, c-kit, CD45). Most recently, it has been shown that a population of progenitor cells can copurify with mesenchymal bone marrow cells and differentiate under specific culture conditions to form both hepatic epithelial and also endothelial cells. The interaction of haemopoietic and mesenchymal stem cells needs further evaluation. The close association of ductular reactive cells and neovessels in end-stage cholestatic liver diseases and the relation to Jagged/Notch signalling pathway may be important in the regulation of stem cells to form both biliary epithelial and endothelial cells.

Delta-1 enhances marrow and thymus repopulating ability of human CD34(+)CD38(-) cord blood cells

We investigated the effect of Notch signaling, a known regulator of cell fate in numerous developmental systems, on human hematopoietic precursors. We show that activation of endogenous Notch signaling in human CD34(+)CD38(-) cord blood precursors with immobilized Delta-1 in serum-free cultures containing fibronectin and hematopoietic growth factors inhibited myeloid differentiation and induced a 100-fold increase in the number of CD34(+) cells compared with control cultures. Immobilized Delta-1 also induced a multifold expansion of cells with the phenotype of common lymphoid precursors (CD34(+)CD7(+)CD45RA(+)) and promoted the development of cytoplasmic CD3(+) T/NK cell precursors. IL-7 enhanced the promotion of T/NK cell differentiation by immobilized Delta-1, but granulocytic differentiation occurred when G-CSF was added. Transplantation into immunodeficient mice showed a substantial increase in myeloid and B cell engraftment in the marrow and also revealed thymic repopulation by CD3(+) T cells due to cells being cultured for a longer period with immobilized Delta-1. These data suggest that Delta-1 can enhance myeloid and lymphoid marrow-repopulating ability and promote the generation of thymus-repopulating T cell precursors.

Active form of Notch imposes T cell fate in human progenitor cells

The crucial role of Notch signaling in cell fate decisions in hematopoietic lineage and T lymphocyte development has been well established in mice. Overexpression of the intracellular domain of Notch mediates signal transduction of the protein. By retroviral transduction of this constitutively active truncated intracellular domain in human CD34+ umbilical cord blood progenitor cells, we were able to show that, in coculture with the stromal MS-5 cell line, depending on the cytokines added, the differentiation toward CD19+ B lymphocytes was blocked, the differentiation toward CD14+ monocytes was inhibited, and the differentiation toward CD56+ NK cells was favored. The number of CD7+cyCD3+ cells, a phenotype similar to T/NK progenitor cells, was also markedly increased. In fetal thymus organ culture, transduced CD34+ progenitor cells from umbilical cord blood cells or from thymus consistently generated more TCR-gammadelta T cells, whereas the other T cell subpopulations were largely unaffected. Interestingly, when injected in vivo in SCID-nonobese diabetic mice, the transduced cells generated ectopically human CD4+CD8+ TCR-alphabeta cells in the bone marrow, cells that are normally only present in the thymus, and lacked B cell differentiation potential. Our results show unequivocally that, in human, Notch signaling inhibits the monocyte and B cell fate, promotes the T cell fate, and alters the normal T cell differentiation pathway compatible with a pretumoral state.

Notch signaling in hematopoiesis and early lymphocyte development

Notch signals regulate multiple cell fate decisions during metazoan development. During hematopoiesis, Notch affects both hematopoietic stem cells and committed progenitors. In hematopoietic stem cells, Notch signaling has the propensity to expand the stem cells, promote their self-renewal, and influence their survival. In committed progenitors, Notch signaling plays a key role in determining lymphoid cell fates. This review focuses on recent developments to understand the role of Notch signaling in early events in hematopoiesis.

Lineage plasticity and commitment in T-cell development

The earliest stages of intrathymic T-cell development include not only the acquisition of T-cell characteristics but also programmed loss of potentials for B, natural killer, and dendritic cell development. Evidence from genetics and cell-transfer studies suggests an order and some components of the mechanisms involved in loss of these options, but some of the interpretations conflict. The conflicts can be resolved by a view that postulates overlapping windows of developmental opportunity and individual mechanisms regulating progression along each pathway. This view is consistent with molecular evidence for the expression patterns of positive regulators of non-T developmental pathways, SCL, PU.1 and Id2, in early thymocytes. To some extent, overexpression of such regulators redirects thymocyte development in vitro. Specific commitment functions may normally terminate this developmental plasticity. Both PU.1 overexpression and stimulation of ectopically expressed growth factor receptors can perturb T- and myeloid/dendritic-cell divergence, but only in permissive stages. A cell-line system that approximates DN3-stage thymocytes reveals that PU.1 can alter specification even in a homogeneous population. However, the response of the population to PU.1 is sharply discontinuous. These studies show a critical role for regulatory context in restricting plasticity, which is probably maintained by interacting transcription factor networks.

Notch signaling as a therapeutic target

Signals transduced by Notch receptors influence differentiation and proliferation in a wide variety of cell types. Activation of a Notch signal by one of several ligands triggers a series of proteolytic cleavages that release the intracellular region of Notch from the membrane, allowing it ultimately to translocate to the nucleus and activate the transcription of downstream target genes. Recent studies have elucidated the roles of several key proteins that participate in and modulate these central events in Notch signal transduction. These advances offer a variety of potential avenues to manipulate Notch signaling for therapeutic purposes in the treatment of cancer and in stem cell maintenance.

Hematologic Aspects of HIV/AIDS

This review addresses various aspects of HIV infection pertinent to hematology, including the consequences of HIV infection on specific aspects of hematopoiesis and an update on the current biologic, epidemiologic and therapeutic aspects of AIDS-related lymphoma and Hodgkin's disease. The results of the expanding use of progenitor cell transplantation in HIV infected patients are also reviewed. In Section I, Dr. Scadden reviews the basis for HIV dysregulation of blood cell production, focusing on the role of the stem cell in HIV disease. T cell production and thymic function are discussed, with emphasis placed upon the mechanisms of immune restoration in HIV infected individuals. Results of clinical and correlative laboratory studies are presented. In Section II, Dr. Levine reviews the recent epidemiologic trends in the incidence of lymphoma, since the widespread availability of highly active anti-retroviral therapy (HAART). The biologic aspects of AIDS-lymphoma and Hodgkin's disease are discussed in terms of pathogenesis of disease. Various treatment options for these disorders and the role of concomitant anti-retroviral and chemotherapeutic intervention are addressed. Drs. Zaia and Krishnan will review the area of stem cell transplantation in patients with AIDS related lymphoma, presenting updated information on clinical results of this procedure. Additionally, they report on the use of gene therapy, with peripheral blood CD34+ cells genetically modified using a murine retrovirus, as a means to treat underlying HIV infection. Results of gene transfer experiments and subsequent gene marking in HIV infected patients are reviewed.

Haematopoietic stem cells

Considerable efforts have been made in recent years in determining the composition of the cell types that constitute the human haematopoietic stem cell (HSC) compartment. These studies have emphasized the heterogeneity of the human HSC in terms of proliferative and self-renewal capacities. Recent studies have indicated that CD34 is not the universal marker of all human HSCs. New markers for purifying HSCs have been described. A number of genes that regulate the formation, self-renewal, or differentiation of HSCs has been identified. The elucidation of the molecular phenotype of the HSC has just begun. Finally, an unexpected degree of developmental or differentiation plasticity of HSC has emerged. This review summarizes all the recent advances made in the human HSC field and examines the impacts that these discoveries may have both clinically and in understanding the organization of the human haematopoietic system.

Hematopoietic progenitor/stem cells immortalized by Lhx2 generate functional hematopoietic cells in vivo

Hematopoietic stem cells (HSCs) are unique in their capacity to maintain blood formation following transplantation into immunocompromised hosts. Expansion of HSCs in vitro is therefore important for many clinical applications but has met with limited success because the mechanisms regulating the self-renewal process are poorly defined. We have previously shown that expression of the LIM-homeobox gene Lhx2 in hematopoietic progenitor cells derived from embryonic stem cells differentiated in vitro generates immortalized multipotent hematopoietic progenitor cell lines. However, HSCs of early embryonic origin, including those derived from differentiated embryonic stem cells, are inefficient in engrafting adult recipients upon transplantation. To address whether Lhx2 can immortalize hematopoietic progenitor/stem cells that can engraft adult recipients, we expressed Lhx2 in hematopoietic progenitor/stem cells derived from adult bone marrow. This approach allowed for the generation of immortalized growth factor-dependent hematopoietic progenitor/stem cell lines that can generate erythroid, myeloid, and lymphoid cells upon transplantation into lethally irradiated mice. When transplanted into stem cell-deficient mice, these cell lines can generate a significant proportion of circulating erythrocytes in primary, secondary, and tertiary recipients for at least 18 months. Thus, Lhx2 immortalizes multipotent hematopoietic progenitor/stem cells that can generate functional progeny following transplantation into lethally irradiated hosts and can long-term repopulate stem cell-deficient hosts.

The notch pathway: modulation of cell fate decisions in hematopoiesis

The hematopoietic system is maintained by a rare population of hematopoietic stem cells (HSC) that are thought to undergo self-renewal as well as continuously produce progeny that differentiate into the various hematopoietic lineages. However, the mechanisms regulating cell fate choices by HSC and their progeny have not been understood. Results of most studies support a stochastic model of cell fate determination in which growth factors support only the survival or proliferation of the progeny specified along a particular lineage. In other developmental systems, however, Notch signaling has been shown to play a central role in regulating fate decisions of numerous types of precursors, often inhibiting a particular (default) pathway while permitting self-renewal or differentiation along an alternative pathway. There is also accumulating evidence that the Notch pathway affects survival, proliferation, and cell fate choices at various stages of hematopoietic cell development, including the decisions of HSC to self-renew or differentiate and of common lymphoid precursors to undergo T- or B-cell differentiation. These data suggest that the Notch pathway plays a fundamental role in the development and maintenance of the hematopoietic system.

Elements of transcription factor network design for T-lineage specification

The complex spectrum of cell types produced in mammalian hematopoiesis can be understood as the output of highly combinatorial transcription factor action. The generation of multiple diverse combinations of transcription factors from the common starting state of the hematopoietic stem cell must be explained through the cross-regulatory interactions of these transcription factors at several levels. Here, the operation of such a network is addressed through a focus on murine T cell development, where we have recently established regulatory linkages between GATA-3 and PU.1 and multiple other factors essential to this differentiation pathway. The action of both essential/rate-limiting factors and factors with effects that shift qualitatively with dose and time of action can be traced through the regulatory interaction network. Hypothetical models are proposed to indicate the network nodes that are differentially activated in normal T cell lineage progression and in cells diverted to other potential fates.

Cell cycle regulators and hematopoiesis

The cell cycle behavior of hematopoietic cells varies from extended quiescence to spectacular proliferation. Cell cycle regulators choreograph these transitions through variation in the makeup of cyclin-dependent kinase (cdk)-containing complexes and through alteration in protein expression levels and subcellular localization. The mechanisms through which cell cycle regulators couple proliferation, differentiation and survival is coming into sharper focus. Cdk-inhibitors, once thought of solely in terms of a checkpoint function on cycling, are now known to interact directly with proteins and pathways central to differentiation and apoptosis. By shuttling between binding partners committed to discrete functional pathways, cell cycle regulators may directly coordinate proliferation with differentiation, migration and apoptosis.

Hematopoietic cytokines, transcription factors and lineage commitment

The past two decades have witnessed significant advances in our understanding of the cellular physiology and molecular regulation of hematopoiesis. At the heart of stem cell self-renewal and lineage commitment decisions lies the relative expression levels of lineage-specific transcription factors. The expression of these transcription factors in early stem cells may be promiscuous and fluctuate, but ultimately comes under the influence of extracellular regulatory signals in the form of hematopoietic cytokines. In this review, we first summarize our current understanding of the phenotypic characterization of hematopoietic stem cells. Next, we describe key known transcription factors which govern stem cell self-renewal and lineage commitment decisions. Finally, we review data concerning the role of specific cytokines in influencing these decisions. From this review, a picture emerges in which stem cell fate decisions are governed by the integrated effects of intrinsic transcription factors and external signaling pathways initiated by regulatory cytokines.

Activated Notch1 signaling promotes tumor cell proliferation and survival in Hodgkin and anaplastic large cell lymphoma

Notch signaling controls cell fate decisions of hematopoietic progenitors by inhibiting certain steps of differentiation and inducing either self-renewal or differentiation toward lymphoid or myeloid lineages. In addition, truncated Notch1 alleles could be associated with 10% of all cases of human T lymphoblastic leukemia and, when introduced into mouse bone marrow stem cells, cause T-cell neoplasms. However, functional links between the abundant expression of intact Notch1 and oncogenesis are still lacking. Here we show that Notch1 is highly expressed in B- and T-cell-derived tumor cells of Hodgkin and anaplastic large cell lymphoma. We demonstrate a novel mechanism for the oncogenic capacity of Notch1 by showing that the interaction between intact Notch1 on tumor cells and its ligand Jagged1 dramatically induces proliferation and inhibition of apoptosis in vitro. We further provide evidence that in Hodgkin and anaplastic large cell lymphoma, Jagged1 is expressed in malignant and in bystander cells colocalizing with Notch1-positive tumor cells. Notch1 signaling may therefore be activated in tumor cells by Jagged1 through homotypic or heterotypic cell-cell interactions, and it seems likely that these interactions contribute to lymphomagenesis in vivo. Thus, our data suggest that activated Notch1 signaling plays an important role in the pathobiology of Hodgkin and anaplastic large cell lymphoma and that it might be a potential new target for treatment.

C-terminal truncation of WT1 delays but does not abolish hematopoiesis in embryoid bodies

The effect of mutations that truncate the WT1 protein on in vitro hematopoietic differentiation from embryonal stem cells has been examined by CFU-A assay, o-dianisidine staining for heme, and RT-PCR analysis of the expression of fetal and adult globins. In two independently isolated ES cell lines the mutations delay but do not abolish hematopoiesis. Analysis of replated CFU-A colonies indicates that the delay occurs prior to the formation of hematopoietic stem cells. The results demonstrate a role for WT1 at the onset of hematopoiesis.

JAK2, complemented by a second signal from c-kit or flt-3, triggers extensive self-renewal of primary multipotential hemopoietic cells

Defining signals that can support the self-renewal of multipotential hemopoietic progenitor cells (MHPCs) is pertinent to understanding leukemogenesis and may be relevant to developing stem cell-based therapies. Here we define a set of signals, JAK2 plus either c-kit or flt-3, which together can support extensive MHPC self-renewal. Phenotypically and functionally distinct populations of MHPCs were obtained, depending on which receptor tyrosine kinase, c-kit or flt-3, was activated. Self-renewal was abrogated in the absence of STAT5a/b, and in the presence of inhibitors targeting either the mitogen-activated protein kinase or phosphatidylinositol 3' kinase pathways. These findings suggest that a simple two-component signal can drive MHPC self-renewal.

Gli and hedgehog in cancer: tumours, embryos and stem cells

Do tumours arise from stem cells, or are they derived from more differentiated cells that, for some reason, begin to recapitulate developmental programmes? Inappropriate activation of the Sonic hedgehog-Gli signalling pathway occurs in several types of tumour, including those of the brain and the skin. Studies in these and other systems suggest that inappropriate function of the Gli transcription factors in stem or precursor cells might lead to the onset of a tumorigenic programme and that these factors are prime targets for anticancer therapies.

Cell fate determination from stem cells

In the adult, tissue-specific stem cells are thought to be responsible for the replacement of differentiated cells within continuously regenerating tissues, such as the liver, skin, and blood system. In this review, we will consider the factors that influence stem cell fate, taking as a primary example the cell fate determination of hematopoietic stem cells.

HOXB4-induced expansion of adult hematopoietic stem cells ex vivo

Hox transcription factors have emerged as important regulators of primitive hematopoietic cell proliferation and differentiation. In particular, HOXB4 appears to be a strong positive regulator of hematopoietic stem cell (HSC) self-renewal. Here we demonstrate the potency of HOXB4 to enable high-level ex vivo HSC expansion. Cultures of nontransduced or GFP-transduced murine bone marrow cells experienced large HSC losses over 10-14 days. In sharp contrast, cultures of HOXB4-transduced cells achieved rapid, extensive, and highly polyclonal HSC expansions, resulting in over 1000-fold higher levels relative to controls and a 40-fold net HSC increase. Importantly, these HSCs retained full lympho-myeloid repopulating potential and enhanced in vivo regenerative potential, demonstrating the feasibility of achieving significant ex vivo expansion of HSCs without functional impairment.

Deciphering the role of Notch signaling in lymphopoiesis

Components of the Notch signaling pathway are expressed during multiple stages of lymphoid development. Consistent with its function during invertebrate development, Notch signaling is proposed to have a central role in lymphoid cell-fate specification. Recent studies show that Notch signaling is a proximal event in T-cell commitment from a common lymphoid progenitor. The role of Notch at later stages of lymphoid development is controversial, but recent data suggest models that may help clarify observations. Current studies suggest that Notch activity is cell-context dependent and interactions between Notch and other environmental receptors are integrated during cell-fate decisions. Furthermore, the requirement for precise regulation of Notch activity is evident from human and murine neoplasms in which dysregulated Notch signaling leads to T-cell leukemia. Future studies that identify the stages of lymphoid development where Notch signaling is physiologically active and the exact targets of Notch signaling that are relevant to lymphopoiesis should significantly improve our understanding of Notch function in T- and B-cell development.

Notch1 activation increases hematopoietic stem cell self-renewal in vivo and favors lymphoid over myeloid lineage outcome

Hematopoietic stem cells sequentially pass through a series of decision points affecting self-renewal or lineage-specific differentiation. Notch1 receptor is a known modulator of lineage-specific events in hematopoiesis that we assessed in the context of in vivo stem cell kinetics. Using RAG-1(-/-) mouse stems cells, we documented increased stem cell numbers due to decreased differentiation and enhanced stem cell self-renewal induced by Notch1. Unexpectedly, preferential lymphoid over myeloid lineage commitment was noted when differentiation occurred. Therefore, Notch1 affects 2 decision points in stem cell regulation, favoring self-renewal over differentiation and lymphoid over myeloid lineage outcome. Notch1 offers an attractive target for stem cell manipulation strategies, particularly in the context of immunodeficiency and acquired immunodeficiency syndrome.

An invitation to T and more: notch signaling in lymphopoiesis

Cell fate decisions in metazoans are regulated by Notch signals. During lymphoid development, Notch influences a series of cell fate decisions involving multipotent progenitors. This review focuses on current views and lingering uncertainties about Notch function in lymphoid cells.

The soluble Notch ligand, Jagged-1, inhibits proliferation of CD34+ macrophage progenitors

The Notch/Notch ligand system controls diverse cellular processes. The proteolytic cleavage generates transmembrane and soluble forms of Notch ligands. We examined the effect of a soluble Notch ligand, human Jagged-1, on human cord blood (CB) CD34+ cells, under serum-deprived conditions, using soluble human Jagged-1-immunoglobulin G1 chimera protein (hJagged-1). Soluble hJagged-1 inhibited myeloid colony formation but not erythroid-mix or erythroid colony formation, in the presence of stem cell factor (SCF), interleukin-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, thrombopoietin, and erythropoietin. Cytological analysis revealed that the decrease in myeloid colonies resulted mainly from the inhibition of macrophage colony formation. Furthermore, soluble hJagged-1 led to the inhibition of macrophage colony formation supported by M-CSF plus SCF and GM-CSF plus SCF. Delayed-addition experiments and the analysis of colony sizes demonstrated that soluble hJagged-l inhibited the growth of macrophage progenitors by acting in the early stage of macrophage development. The direct action of hJagged-1 was confirmed by the enhanced expression of the HES-1 (hairy enhancer of the split-1) gene. These results suggest that soluble hJagged-1 may regulate human hematopoiesis in the monocyte/macrophage lineage.

Differential gene expression profiling of adult murine hematopoietic stem cells

Hematopoietic stem cells (HSCs) have self-renewal capacity and multilineage developmental potentials. The molecular mechanisms that control the self-renewal of HSCs are still largely unknown. Here, a systematic approach using bioinformatics and array hybridization techniques to analyze gene expression profiles in HSCs is described. To enrich mRNAs predominantly expressed in uncommitted cell lineages, 54 000 cDNA clones generated from a highly enriched population of HSCs and a mixed population of stem and early multipotent progenitor (MPP) cells were arrayed on nylon membranes (macroarray or high-density array), and subtracted with cDNA probes derived from mature lineage cells including spleen, thymus, and bone marrow. Five thousand cDNA clones with very low hybridization signals were selected for sequencing and further analysis using microarrays on glass slides. Two populations of cells, HSCs and MPP cells, were compared for differential gene expression using microarray analysis. HSCs have the ability to self-renew, while MPP cells have lost the capacity for self-renewal. A large number of genes that were differentially expressed by enriched populations of HSCs and MPP cells were identified. These included transcription factors, signaling molecules, and previously unknown genes.

The notch receptor and its ligands are selectively expressed during hematopoietic development in the mouse

Members of the Notch family of transmembrane receptors are found on primitive hematopoietic precursors, and Notch ligand expression has been demonstrated on the surface of stromal cells, suggesting a role for Notch signaling in mammalian blood cell development. The current report examines the expression of Notch receptors and their ligands in murine hematopoietic tissues to determine: A) which blood cell lineages in the adult are influenced by Notch activity, and B) whether fetal hematopoiesis in the embryo involves the Notch pathway. In the adult mouse, a combination of flow cytometry, immunohistochemistry and Northern analysis was used to examine Notch receptor or ligand expression in bone marrow and spleen. In the embryo, Northern analysis and in situ hybridization were used to characterize Notch receptor and ligand expression in fetal liver on embryonic day 12 (E12) through E17, an active period encompassing both erythropoiesis and granulopoeisis. Flow cytometry demonstrated the presence of Notch1 and Notch2 receptors on bone marrow-derived myeloid cells but not on erythroid cells positive for the marker, Ter-119. In situ hybridization of E12 through E17 fetal liver demonstrated widespread expression of Jagged1 and Delta1 in a pattern similar to but less abundant than that of the erythropoietin receptor. Taken together with earlier functional results, the current expression data suggest a role for Notch activity in establishing definitive hematopoiesis in fetal liver, as well as a selective use of Notch signaling in adult erythropoiesis and granulopoiesis. Notch receptors in the adult are most likely utilized by early erythroid precursors and intermediate-stage granulocytes, but not by terminally differentiating cells of either subset.

Notch1 inhibits differentiation of hematopoietic cells by sustaining GATA-2 expression

Notch signaling is involved in cell fate decisions in many systems including hematopoiesis. It has been shown that expression of an activated form of Notch1 (aNotch1) in 32D mouse myeloid progenitor cells inhibits the granulocytic differentiation induced by granulocyte colony-stimulating factor (G-CSF). Results of the current study show that aNotch1, when expressed in F5-5 mouse erythroleukemia cells, also inhibits erythroid differentiation. Comparison of the expression levels of several transcription factors after stimulation for myeloid and erythroid differentiation, in the presence or absence of aNotch1, revealed that aNotch1 did not change its regulation pattern with any of the transcription factors examined, except for GATA-2, despite its inhibitory effect on differentiation. GATA-2 was down-regulated when the parental 32D and F5-5 were induced to differentiate into granulocytic and erythroid lineages, respectively. In these induction procedures, however, the level of GATA-2 expression was sustained when aNotch1 was expressed. To ascertain whether maintenance of GATA-2 is required for the Notch-induced inhibition of differentiation, the dominant-negative form of GATA-3 (DN-GATA), which acted also against GATA-2, or transcription factor PU.1, which was recently shown to be the repressor of GATA-2, was introduced into aNotch1-expressing 32D (32D/aNotch1) cells that do not express GATA family proteins other than GATA2. Both DN-GATA and PU.1 reversed the phenotype of 32D/aNotch1 inducing its differentiation when G-CSF was added. Furthermore, enforced expression of HES-1, which is involved in Notch signaling, delayed differentiation of 32D, and again this phenotype was neutralized by DN-GATA. These results indicate that GATA-2 activity is necessary for the Notch signaling in hematopoietic cells.

Phosphorylation of Ser2078 modulates the Notch2 function in 32D cell differentiation

Notch signaling is involved in the regulation of many cell fate determination events in both embryonic development and adult tissue homeostasis. We previously demonstrated that Notch1 and Notch2 molecules inhibit myeloid differentiation in a cytokine-specific manner and that the Notch cytokine response domain is necessary for this functional specificity. We have now investigated the putative role of phosphorylation in the activity of Notch in response to cytokine signals. Our results show that the granulocyte colony-stimulating factor (G-CSF) stimulation of 32D cells expressing the intracellular Notch2 protein induces phosphorylation at specific sites of this molecule, rendering the molecule inactive and permitting differentiation of these cells. In contrast, when cells are stimulated with granulocyte macrophage colony-stimulating factor (GM-CSF), intracellular notch2 is not phosphorylated at these residues and differentiation is inhibited. We also show that deletion of the Ser/Thr-rich region between amino acids 2067 and 2099 abrogates G-CSF-induced phosphorylation and results in a molecule that inhibits differentiation in response to either G-CSF or GM-CSF. Our results further indicate that Ser(2078) is a critical residue for phosphorylation and modulation of Notch2 activity in the context of G-CSF-induced differentiation of 32D cells.

Age- and stage-specific regulation patterns in the hematopoietic stem cell hierarchy

The molecular mechanisms that regulate self-renewal and differentiation of very primitive hematopoietic stem and progenitor cells in vivo are still poorly understood. Despite the clinical relevance, even less is known about the mechanisms that regulate these cells in old animals. In a forward genetic approach, using quantitative trait linkage analysis in the mouse BXD recombinant inbred set, this study identified loci that regulate the genetic variation in the size of primitive hematopoietic cell compartments of young and old C57BL6 and DBA/2 animals. Linked loci were confirmed through the generation and analysis of congenic animals. In addition, a comparative linkage analysis revealed that the number of primitive hematopoietic cells and hematopoietic stem cells are regulated in a stage-specific and an age-specific manner.

Stem cells, cancer, and cancer stem cells

Stem cell biology has come of age. Unequivocal proof that stem cells exist in the haematopoietic system has given way to the prospective isolation of several tissue-specific stem and progenitor cells, the initial delineation of their properties and expressed genetic programmes, and the beginnings of their utility in regenerative medicine. Perhaps the most important and useful property of stem cells is that of self-renewal. Through this property, striking parallels can be found between stem cells and cancer cells: tumours may often originate from the transformation of normal stem cells, similar signalling pathways may regulate self-renewal in stem cells and cancer cells, and cancer cells may include 'cancer stem cells' - rare cells with indefinite potential for self-renewal that drive tumorigenesis.

Differential effects of Notch ligands Delta-1 and Jagged-1 in human lymphoid differentiation

Notch signaling is known to differentially affect the development of lymphoid B and T cell lineages, but it remains unclear whether such effects are specifically dependent on distinct Notch ligands. Using a cell coculture assay we observed that the Notch ligand Delta-1 completely inhibits the differentiation of human hematopoietic progenitors into the B cell lineage while promoting the emergence of cells with a phenotype of T cell/natural killer (NK) precursors. In contrast, Jagged-1 did not disturb either B or T cell/NK development. Furthermore, cells cultured in the presence of either Delta-1 or Jagged-1 can acquire a phenotype of NK cells, and Delta-1, but not Jagged-1, permits the emergence of a de novo cell population coexpressing CD4 and CD8. Our results thus indicate that distinct Notch ligands can mediate differential effects of Notch signaling and provide a useful system to further address cell-fate decision processes in lymphopoiesis.

Notch signaling in lymphocyte development

Signaling through Notch has been implicated in many cell-fate decisions during lymphocyte development. Recent studies have provided new clues--and raised new controversies--regarding the exact role that Notch signaling plays in the commitment of cells to the T-cell lineage. Progress has also been made in deducing the transcriptional program induced by Notch and the mechanism of oncogenic transformation by Notch in lymphocytes.

Notch receptors and hematopoiesis

Notch receptors are involved in a variety of cell-fate decisions that affect the development and function of many organs, including hematopoiesis and the immune system. There are four mammalian Notch receptors that have only partially overlapping functions despite sharing similar structures and ligands. The ligands for Notch are transmembrane proteins expressed on adjacent cells, including Jagged and Delta, and it is quite possible that signaling is bidirectional. A large Notch precursor protein is proteolytically cleaved to form the mature cell-surface receptor. Ligand binding induces additional proteolytic events followed by translocation of the intracellular domain to the nucleus. There, Notch interacts with transcription factors such as RBPJ kappa, activating transcription of basic helix-loop-helix genes such as HES1. These in turn regulate expression of tissue-specific transcription factors that influence lineage commitment and other events. In this review, the details of Notch signaling will be discussed, with a focus on what is known about the role of Notch in hematopoiesis.

Controlling culture dynamics for the expansion of hematopoietic stem cells

The ex vivo expansion of hematopoietic stem cells (HSCs) is the subject of intense commercial and academic interest due to the potential of HSCs to be a renewable source of material for cellular therapeutics. Unfortunately, because methodologies have not yet been developed to grow clinically relevant numbers of HSCs (or their derivatives) consistently, the potential of this technology is limited. Manipulation of the in vitro culture microenvironment, primarily through cytokine supplementation, has been the predominant approach in studies attempting to expand primary human HSC numbers in vitro. While promising results have been obtained, it is becoming clear that novel methods must be developed before cellular therapies using these stem cells can become routine. Ideally, bioprocesses must be designed to target specifically the growth of stem cell populations while incorporating positive and negative feedback from potentially dynamic mature and maturing cell populations. The product of these culture systems should consist of not only HSCs, but also of cells that allow the engraftment of HSCs and, ideally, cells responsible for the immediate or accelerated functional support of patients. Development of such "designer transplants" will require combining optimal culture conditions capable of amplifying HSC numbers with novel approaches for finely controlling the number, functional capabilities, and characteristics of potentially therapeutic cells in these very complex cell culture systems.

Presenilin-dependent gamma-secretase activity modulates thymocyte development

In neuronal cells, presenilin-dependent gamma-secretase activity cleaves amyloid precursor proteins to release Abeta peptides, and also catalyzes the release of the intracellular domain of the transmembrane receptor Notch. Accumulation of aberrant Abeta peptides appears to be causally related to Alzheimer's disease. Inhibition of Abeta peptide production is therefore a potential target for therapeutic intervention. Notch proteins play an important role in cell fate determination in many different organisms and at different stages of development, for example in mammalian T cell development. We therefore addressed whether structurally diverse gamma-secretase inhibitors impair Notch function by studying thymocyte development in murine fetal thymic organ cultures. Here we show that high concentrations of the most potent inhibitors blocked thymocyte development at the most immature stage. In contrast, lower concentrations or less potent inhibitors impaired differentiation at a later stage, most notably suppressing the development of CD8 single-positive T cells. These phenotypes are consistent with an impairment of Notch signaling by gamma-secretase inhibitors and define a strict Notch dose dependence of consecutive stages during thymocyte development.

Notch signaling in leukemia

Mammalian Notch homologs were first identified from the involvement of Notch1 in a recurrent chromosomal translocation in a subset of human T-cell leukemias. The effect of the translocation was twofold: Notch expression was placed under the control of a T-cell-specific element, and Notch was truncated, resulting in a constitutively active protein. Subsequent work has shown that Notch1 is required for T cell commitment and is exclusively oncotropic for T cells. During the past year, several murine models have been used to dissect the function of Notch signaling in lymphoid development and leukemia. These models show that Notch1 drives the earliest stages of T cell commitment and that Notch signaling must be downregulated by the double positive stage for proper T cell development to occur. Constitutive Notch signaling mediated by Notch1, Notch2, or Notch3 predisposes to T-cell leukemia. Future studies are expected to elucidate the mechanisms by which Notch leads to transformation. Identification of the transcriptional targets of Notch signaling is likely to yield important insights.

Notch1IC partially replaces EBNA2 function in B cells immortalized by Epstein-Barr virus

Immortalization of B cells by Epstein-Barr virus (EBV) depends on the virally encoded EBNA2 protein. Although not related by sequence, the cellular Notch protein and EBNA2 share several biochemical and functional properties, such as interaction with CBF1 and the ability to activate transcription of a number of cellular and viral genes. Whether these similarities are coincidental or exemplify EBNA2 mimicry of evolutionarily conserved cellular signaling pathways is unclear. We therefore investigated whether activated forms of Notch could substitute for EBNA2 in maintaining the immortalized phenotype of EBV-infected B cells. To address this question, we devised a transcomplementation system using EREB2.5 cells. EREB2.5 cells are immortalized by EBV expressing a conditional estrogen receptor EBNA2 fusion protein (EREBNA2), and cellular proliferation is dependent on the availability of estrogen. Withdrawal of estrogen results in inactivation of EREBNA2, leading to growth arrest and eventually to cell death. Transduction of EREB2.5 cells with a lentiviral vector expressing wild-type EBNA2 rescued EREB2.5 cells from the growth-inhibitory effects of estrogen deprivation, in contrast to transduction with the lentivirus vector alone. EREB2.5 cells were also rescued by enforced expression of human Notch1IC after estrogen starvation, but this effect was restricted to cells expressing high levels of the transcription factor. Compared to wild-type EBNA2-expressing EREB2.5 cells, the Notch-expressing cells expanded more slowly after estrogen starvation, and once established, they continued to display a lower proliferation rate. Analysis of viral and cellular gene expression from transduced EREB2.5 cells after estrogen withdrawal indicated that both wild-type EBNA2- and Notch1IC-positive cells expressed c-Myc at levels similar to those found in parental EREB2.5 cells. However, the latter cells expressed LMP-1 far less efficiently than cells transduced with the wild-type EBNA2 gene. Cells rescued by either wild-type EBNA2 or Notch1IC expressed surface CD21 and CD23 proteins, but not CD10, indicating that induction of relevant type III latency markers was maintained. The data imply that both Notch and EBNA2 activate an important subset of cellular genes associated with type III latency and B-cell growth, while EBNA2 more efficiently induces important viral genes, such as LMP-1. Thus, exploitation of conserved Notch-related signaling pathways may represent a key mechanism by which EBNA2 contributes to EBV-induced cell immortalization.

Gamma -secretase inhibitors repress thymocyte development

A major therapeutic target in the search for a cure to the devastating Alzheimer's disease is gamma-secretase. This activity resides in a multiprotein enzyme complex responsible for the generation of Abeta42 peptides, precipitates of which are thought to cause the disease. Gamma-secretase is also a critical component of the Notch signal transduction pathway; Notch signals regulate development and differentiation of adult self-renewing cells. This has led to the hypothesis that therapeutic inhibition of gamma-secretase may interfere with Notch-related processes in adults, most alarmingly in hematopoiesis. Here, we show that application of gamma-secretase inhibitors to fetal thymus organ cultures interferes with T cell development in a manner consistent with loss or reduction of Notch1 function. Progression from an immature CD4-/CD8- state to an intermediate CD4+/CD8+ double-positive state was repressed. Furthermore, treatment beginning later at the double-positive stage specifically inhibited CD8+ single-positive maturation but did not affect CD4+ single-positive cells. These results demonstrate that pharmacological gamma-secretase inhibition recapitulates Notch1 loss in a vertebrate tissue and present a system in which rapid evaluation of gamma-secretase-targeted pharmaceuticals for their ability to inhibit Notch activity can be performed in a relevant context.

Human homologues of Delta-1 and Delta-4 function as mitogenic regulators of primitive human hematopoietic cells

Delta-mediated Notch signaling controls cell fate decisions during invertebrate and murine development. However, in the human, functional roles for Delta have yet to be described. This study reports the characterization of Delta-1 and Delta-4 in the human. Human Delta-4 was found to be expressed in a wide range of adult and fetal tissues, including sites of hematopoiesis. Subsets of immature hematopoietic cells, along with stromal and endothelial cells that support hematopoiesis, were shown to express Notch and both Delta-1 and Delta-4. Soluble forms of human Delta-1 (hDelta-1) and hDelta-4 proteins were able to augment the proliferation of primitive human hematopoietic progenitors in vitro. Intravenous transplantation of treated cultures into immune-deficient mice revealed that hDelta-1 is capable of expanding pluripotent human hematopoietic repopulating cells detected in vivo. This study provides the first evidence for a role of Delta ligands as a mitogenic regulator of primitive hematopoietic cells in the human.

Human homologues of Delta-1 and Delta-4 function as mitogenic regulators of primitive human hematopoietic cells

Delta-mediated Notch signaling controls cell fate decisions during invertebrate and murine development. However, in the human, functional roles for Delta have yet to be described. This study reports the characterization of Delta-1 and Delta-4 in the human. Human Delta-4 was found to be expressed in a wide range of adult and fetal tissues, including sites of hematopoiesis. Subsets of immature hematopoietic cells, along with stromal and endothelial cells that support hematopoiesis, were shown to express Notch and both Delta-1 and Delta-4. Soluble forms of human Delta-1 (h Delta-1) and h Delta-4 proteins were able to augment the proliferation of primitive human hematopoietic progenitors in vitro. Intravenous transplantation of treated cultures into immune-deficient mice revealed that h Delta-1 is capable of expanding pluripotent human hematopoietic repopulating cells detected in vivo. This study provides the first evidence for a role of Delta ligands as a mitogenic regulator of primitive hematopoietic cells in the human. (Blood. 2001;97:1960-1967)