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Review articles

Notch signaling in hematopoiesis and lymphopoiesis: lessons from Freddy Radtke,* Anne Wilson, and H. Robson MacDonald

Summary Introduction The evolutionarily conserved The Notch signaling pathway is highly conserved in evolution regulates a broad spectrum of cell fate decisions and and is found in organisms as diverse as worms and humans. At differentiation processes during fetal and postnatal life. It th is involved in embryonic organogenesis as well as in the the beginning of the 20 century, and maintenance of homeostasis of self-renewing systems. In colleagues described notches at the margin of wing blades of this article, we review the role of Notch signaling in the fruit flies ().(1) These notches were hematopoietic system with particular emphasis on lym- found to be the result of a partial loss of function (haplo- phocyte development and highlight the similarities in Notch function between Drosophila and mammalian insufficiency) of the Notch encoding for a single differentiation processes. Recent studies indicating transmembrane receptor that was cloned in the mid that aberrant NOTCH signaling is frequently linked to 1980s.(2,3) Notch and their corresponding ligands the induction of T leukemia in humans will also be regulate many cell fate decision and differentiation processes discussed. BioEssays 27:1117–1128, 2005. during development.(4) They are also involved in , ß 2005 Wiley Periodicals, Inc. proliferation and border formation. As most of our initial knowledge of the Notch pathway is derived from studies in worms and flies, we will first introduce some basic concepts of Notch function derived from experiments in invertebrates Ludwig Institute for Research, Lausanne Branch, University of before addressing the role of Notch signaling in hematopoiesis Lausanne, Epalinges, Switzerland. and development. These early studies are very *Correspondence to: Freddy Radtke, Ludwig Institute for Cancer Research, Lausanne Branch, University of Lausanne, Chemin des important since interpretation of many of the results obtained Boveresses 155, 1066 Epalinges, Switzerland. in vertebrate organs such as the hematopoietic system are E-mail: [email protected] based on long held concepts established in invertebrates. DOI 10.1002/bies.20315 Published online in Wiley InterScience (www.interscience.wiley.com). Paradigms of Notch function In Drosophila, Notch was shown to influence cell fate choices within a group of cells having equipotent developmental Abbreviations: CNS, central ; PNS, peripheral nervous potential. For example, in the Drosophila central nervous system; SOP sensory organ precursor; NLS, nuclear localization system (CNS), as well as in the peripheral nervous system signal; TACE, tumor-necrosis factor a converting enzyme; NIC, Notch (PNS), Notch signaling specifies a single cell amongst a intracellular domain; CSL, name of the Notch signaling mediating cluster of cells in the (proneural cluster), all with the derived from different species, CBF-1 in humans, (5) Supressor of hairless in Drosophila and Lag1 in Caenorhabditis same potential to become neuroblasts in the CNS or (6,7) elegans, also known as RBP-J in the mouse; Hes, hairy enhancer of sensory organ precursor cells (SOPs) in the PNS. The cell split, Herp, hairy enhancer of split related ; Nrarp, Notch- within the proneural cluster that expresses the highest levels of regulated ankyrin repeat protein; Pre-T-a, pre receptor alpha; Notch ligand triggers Notch signaling in the Notch receptor- MINT, Msx2-interaccting nuclear target protein, YS, yolk sac; AGM, expressing surrounding cells. These Notch signal-receiving aorta-gonad-mesonephros; BM ; HSCs, hematopoietic stem cells; SP, side population; MAML1, mastermind like protein; cells are inhibited from becoming neuroblasts (in the CNS) or TCRb, T cell receptor beta; MZB, marginal zone B cells, FoB, follicular SOPs (in the PNS). Conversely, in the absence of Notch zone B cells; DCs, dendritic cells; T-ALL, T cell actue lymphoblastic signaling, all of the cells within a proneural cluster become leukemia; TAN1, translocation associated Notch homolog, HD, neuroblasts (or SOPs), designated as a neurogenic pheno- heterodimerization domain, ANK, ankyrin repeats, PM, plasma type(8,9) (Fig. 1). Since this cell fate specification process membrane; CR, cysteine-rich, EGF-like, -like, TAD transactivation domain, COR, co-repressors; LP, lymphoid occurs between neighboring cells, it is referred to as lateral progenitors; MP, myeloid progenitors. inhibition or lateral specification. Although these results were interpreted to mean that Notch signaling specifies one

BioEssays 27:1117–1128, ß 2005 Wiley Periodicals, Inc. BioEssays 27.11 1117 Review articles

Figure 1. Notch regulates cell fate specification of the peripheral nervous system in the fruit fly. A: Schematic view of a mechanosensory bristle of the fruit fly. B: The role of Notch during selection of sensory organ precursor cells (SOP) from a proneural cluster and their subsequent cell fate decisions during development of a mechanosensory bristle. Notch signaling controls multiple binary cell fate choices of the SOP cell, the IIa and IIIb progenitors to give rise to a hair, a socket, a and a sheath cell. N, Notch signal receiving cell.

particular cell fate from a pool of cells that can adopt at least formation of sensory bristles. Loss- and gain-of-function two different fates, they also fostered the common view that studies during this differentiation process show that Notch one function of Notch signaling is to inhibit differentiation and signaling is required both at the first cell division to specify the thereby maintain uncommitted precursor cells. In this case, IIa cell formed from a bi-potent SOP,and, in the second round cells not receiving a Notch signal will differentiate, while cells of cell division, for specification of the socket cell from the IIa receiving a Notch signal will not. This view was further cell or the sheath cell from the IIb cell(13) (Fig. 1). These results reinforced by studies in the frog in which injection of RNA showing that Notch signaling regulates the acquisition of coding for the Notch ligand Delta1 into a Xenopus embryo at distinct fates in daughter cells of SOPs led to the more general the two-cell stage inhibited neuron production.(10) In contrast, concept that Notch may control binary cell fate choices from bi- injection of RNA coding for a dominant negative form of Delta1 potent progenitor cells. caused excess production of .(11) Similarly, introduc- A third proposed function of Notch signaling between tion of a dominant active form of Notch into the vertebrate developmentally related cell types would be to induce or P19 cell line (which under certain conditions can differenti- enhance terminal differentiation. This has been shown in both ate into neuronal lineages and muscle progenitors) inhibited human(14) and mouse skin, as well as in primary keratinocytes differentiation.(12) Although these latter experiments are where Notch signaling induces early differentiation markers consistent with the idea that Notch plays a role in inhibition of and cell cycle arrest by Waf1.(15) differentiation of progenitor cells, they are at best suggestive. In summary,Notch signaling can regulate and influence cell Nevertheless they have fostered the common view that Notch fate decisions in at least three different ways: (1) Maintenance functions as progenitor or gate-keeper. of undifferentiated progenitors, (2) regulation of binary cell fate The best-studied role of Notch, however,is the regulation of decisions, and (3) induction of terminal differentiation. binary cell fate choices. A classical example of this is observed in the Drosophila PNS. Once a SOP cell has been specified out Notch receptors and signaling of the proneural cluster in the ectoderm, it divides into two cells Drosophila has one Notch receptor activated by two different termed IIa and IIb. These cells divide a second time to give rise transmembrane-bound ligands called Serrate and Delta, to either hair and socket cells (derived from IIa), or neuron and while (C. elegans) contains two sheath cells (derived from IIb), which participate in the receptors (Glp-1 and Lin-12) and two ligands (Apx-1 and

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Lag-2).(4) In contrast, such as mice and humans include Deltex(30) and Numb(31) while Nrarp (Notch-regulated have four receptors (Notch1-4), and five ligands: Jagged1 and ankyrin repeat protein) and MINT (Msx2-interacting nuclear Jagged2 (homologues of Serrate) and Delta-like 1, 3 and 4 target protein) are nuclear proteins both of which seem to (homologues of Delta) (Fig. 2). Notch receptors are synthe- negatively regulate Notch signaling. How Nrarp inhibits Notch sized as precursor proteins that are cleaved during transport to signaling(32) is currently unknown, whereas MINT has been the cell surface where they are expressed as heterodimers. shown to compete with NIC for binding to CSL.(33) Their extracellular domains contain between 29 and 36 epidermal growth factor-like (EGF) repeats followed by three Notch, a gatekeeper of hematopoietic cysteine-rich Notch/Lin12 (LIN) repeats. While the amino stem cells? terminal EGF-like repeats bind ligand DSL (Delta, Serrate, The wide expression patterns of Notch receptors and their Lag-2) domains, the function of the LIN repeats is to prevent corresponding ligands (reviewed in Ref. 34) within the adult signaling in the absence of ligand. The cytoplasmic domains of hematopoietic system suggest an important role for Notch Notch receptors, which convey the signal to the nucleus, signaling during adult hematopoiesis. Such a role has already harbor multiple protein–protein interaction domains such as been confirmed during embryonic hematopoiesis.(35) The first the RAM domain and the CDC10 repeats, which bind hematopoietic cells in the mouse embryo appear in the downstream effector molecules, two nuclear localization extraembryonic yolk sac (YS) around E7.5, often referred to signals (NLS), and a PEST sequence involved in regulating as primitive hematopoiesis. During development, YS hemato- protein stability. While Notch1 and Notch2 also contain a poiesis shifts, first to a site within the embryo called the aorta– transcriptional transactivation domain, such domains have not gonad–mesonephros (AGM), then to the fetal and finally been described yet for Notch3 and Notch4. Ligand binding to to the adult bone marrow (BM).(36) Hematopoietic stem cells the extracellular domain of Notch receptors triggers two (HSCs) within the AGM region contain long-term repopulating proteolytic cleavages within the receptor. The first is mediated activity as they can reconstitute the entire hematopoietic by the ADAM protease TACE (tumor-necrosis factor a- system of an adult mouse. These fetal HSCs are likely to be converting enzyme), which cleaves the receptors close to derived from a bi-potential hemangioblast that gives rise to the transmembrane domain. The extracellular Notch domain is both endothelial and hematopoietic cells.(37,38) Recently ‘transendocytosed’ by the ligand-expressing neighboring hematopoietic cells have been shown to bud from the cell.(16) A second cleavage, mediated by the g-secretase endothelial cell layer in the murine midgestation dorsal activity of the multiprotein complex of , occurs within aorta,(39) supporting the view that the first long-term repopu- the transmembrane domain. The liberated cytoplasmic lating hematopoietic cells originate from hemangioblasts. Hirai domain (NIC) translocates to the nucleus and binds the and colleagues showed that, while Notch1 signaling is transcription factor CSL (CBF1 in humans, RBP-J in mice, dispensable for primitive hematopoiesis within the extraem- Suppressor of hairless in Drosophila, Lag1 in C. elegans), bryonic YS, it is essential for the reconstitution activity of fetal converting it from a transcriptional repressor into a transcrip- HSCs found in the AGM region.(35) In addition, blocking Notch tional activator by displacing corepressor complexes(17–19) signaling with g-secretase inhibitors (which block the proteo- and recruiting coactivators such as mastermind-like proteins lytic release of NIC) suppresses in vitro hematopoiesis in (MAMLs),(20,21) which interact with CBP/p300 proteins.(22) To explant cultures derived from the E9.5 AGM region, but not date only a few Notch target have been identified, some from the E10.5 AGM region, a time point when HSCs already of which are utilized in multiple tissues while others seem to be exist.(35) During the onset of definitive hematopoiesis in the tissue specific. The basic helix–loop–helix transcription embryo Notch1/CSL signaling activates the expression of factors of the Hairy enhancer of split (Hes) family such as Gata2,(40) which has previously been shown to be an essential Hes1 and Hes5,(23) the related Herp (Hes-related repressor transcription factor for hematopoiesis.(41) These results protein) transcription factor family,(24) the cell cycle regulator strongly suggest that Notch1 signaling is necessary for Cdkn1a,(15) Nrarp (Notch-regulated ankyrin repeat pro- development of HSCs during a very narrow window of time, tein),(25) Deltex1(26) and Ptcra (Pre-T-a)(27) have been re- but not for their maintenance at later stages of embryonic ported as Notch target genes. development. It is therefore tempting to speculate that Notch1 An additional level of complexity stems from the fact that signaling functions in a way analogous to the binary cell fate Notch signaling can be regulated by several modulators that specification of SOP cells by influencing the lineage choice of a act at extracellular, cytoplasmic or nuclear levels. Fringe bi-potential (Fig. 1). If this were true, the bi- proteins belong to a family of glycosyl transferases that add potential progenitor would be the hemangioblast that has to N-acetylglucosamine to certain EGF-like repeats of Notch choose between hematopoietic and endothelial cell fates. In receptors, thereby promoting Notch signaling in response to the absence of Notch1 signaling, hemangioblasts would fail to Delta ligands and inhibiting Jagged-mediated signaling.(28,29) adopt the hematopoietic cell fate and would differentiate into Examples of cytoplasmic modulators of Notch signaling endothelial cells by default (Fig. 3). Although endothelial cell

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Figure 2. Notch receptors/ligands and signaling. A: Notch receptors and ligands. While Drosophila has one Notch receptor (dNotch), vertebrates have four (Notch1–4). The extracellular domain of the receptors contain between 29 and 36 EGF-like repeats (36 in dNotch, Notch1 and 2, 34 in Notch3 and 29 in Notch4) involved in ligand binding, followed by three cysteine-rich Notch/LIN12 (LIN) repeats. The LIN domain prevents ligand-independent activation of the receptor,and is followed by the heterodimerization domain (HD). The cytoplasmic part of the receptor contains two protein–protein interaction domains, the RAM (R) domain, six ankyrin repeats (ANK), two nuclear localization signals (NLS), a transcriptional transactivation domain (TAD) and a PESTsequence. dNotch can be activated by two different ligands, Delta and Serrate while the vertebrate receptors can be activated by at least five ligands (Jagged1 and 2 (homologs of Serrate), and Delta1, 3 and 4 (homologs of Delta)). The common feature of these two ligand families is an N-terminal structure called DSL (Delta, Serrate and Lag). Both type of ligands contain EGF-like repeats in the extracellular domain, but only Serrate, Jagged1 and Jagged2 harbor an additional cysteine- rich (CR) sequence downstream of the EGF-like repeats. PM, plasmamembrane. B) Notch signaling. Notch receptors are synthesized as precursor proteins and cleaved in two during transport to the cell surface where they are expressed as heterodimers. Notch signaling is initiated by ligand binding which induces two subsequent proteolytic cleavages, the first mediated by TACE (tumor-necrosis factor a- converting enzyme) near the transmembrane domain, while the second cleavage which occurs within the transmembrane domain is mediated by the g-secretase activity of presenilins. The liberated cytoplasmic portion of the receptor (NIC) translocates to the nucleus and heterodimerizes with the transcription factor CSL. Binding of NIC to CSL leads to transcriptional activation by displacement of co-repressors (COR) and simultaneous recruitment of coactivators such as Mastermind-like proteins (MAML) that interact with P300.

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Figure 3. Notch triggered cell fate specification within the hematopoietic system. A schematic model analogous to Figure1 showing how Notch influences multiple, potentially binary cell fate decisions during hematopoietic and lymphocyte development. This includes the generation of hematopoietic stem cells (HSCs) within the aorta-gonad-mesonephros (AGM) region of the developing embryo, the T versus B lineage decision of lymphoid progenitors (LP) and ab versus gd Tcell development within the as well as marginal zone B (MZB) versus follicular B (FoB) cell development within the spleen. N (Notch signal receiving cell), BM (bone marrow), MP (myeloid progenitor).

fate specification appears to occur independently of Notch in knocked down Notch signaling in vitro using either a retroviral this particular instance, it is clear that Notch plays other transduction approach to overexpress dominant negative essential roles during vasculogenesis (reviewed in Ref. 42). forms of CSL or the transcriptional co-activator MAML1, or Whether Notch signaling is also important for the cell fate by using g-secretase-specific inhibitors. All three approaches specification of HSCs in adult BM is currently unknown. resulted in enhanced differentiation of HSCs in vitro. In other in However, several lines of evidence support the possibility that vivo experiments, HSCs expressing a dominant negative CSL Notch signaling might be necessary for adult HSC main- were unable to reconstitute the long-term HSC pool in trans- tenance. First, expression of a dominant active form of the planted recipients.(53) Despite this large body of evidence, Notch receptor (NIC) in murine BM progenitors can lead to which is mostly interpreted according to the idea that Notch increased self-renewal in vivo,(43) or to signaling inhibits differentiation and is therefore necessary immortalization of hematopoietic progenitor cells with mye- for HSC maintenance, there is no clear-cut genetic loss of loid and lymphoid differentiation potential.(44) Second, co- Notch function model supporting this hypothesis. In particular, culture assays in which murine or human HSCs are incubated neither conditional inactivation of the CSL gene, which in the presence of immobilized or soluble Notch ligands, or mediates Notch signaling of all Notch receptors, nor condi- together with ligand-expressing feeder cells can maintain or tional loss of function of the Notch1 or the Notch2 genes in even enhance HSC self-renewal.(45–51) This latter process is adult BM cells result in any HSC phenotype, even in stringent possibly mediated by the Notch target gene Hes1, since Hes1 repopulation assays where BM cells deficient for Notch gain-of-function studies have shown maintenance of HSCs signaling components have to compete with wild-type cells. ex vivo. In addition, transplantation of HSCs overexpressing Recently, maintenance of BM HSC numbers has been Hes1 resulted in increased numbers of cells with ‘‘side postulated to be mediated by Jagged1-expressing osteoblasts population’’ (SP) activity, characterized by the active efflux of located in the BM stem cell niche. Osteoblast-specific the DNA dye Hoechst 33342. Long-term hematopoietic re- expression of the activated parathyroid hormone-related populating activity has been shown to be concentrated within protein receptor resulted in an increase in the number of BM SP cells.(52) More recently, Reya and colleagues have osteoblasts, which correlated with an increase in the number

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of functional HSCs.(54) Since osteoblasts showed evidence of Fringe,(68) Deltex(69) or Nrarp,(32) or a dominant negative form Notch1 activation in vivo,(54) these results were interpreted to of the transcriptional coactivator MAML.(70) Expression of any mean that Jagged1-expressing osteoblasts regulate HSC one of these modulators in either BM progenitors or immature homeostasis through Notch1 signaling. However, conditional also results in a block of Tcell development as well inactivation of the Jagged1 gene in BM progenitors and/or BM as in ectopic thymic development. Surprisingly over- stromal cells does not impair HSC self-renewal or differentia- expression of the Numb gene (a negative modulator of the tion into any lineage,(55) suggesting that Jagged1 is not Notch pathway in Drosophila) had no effect on T cell essential for HSC hematopoiesis. development.(71) Although overexpression of these modifiers Although physiological roles for Notch signaling in adult can interfere with Notch signaling, their physiological role HSC maintenance and/or self-renewal appear rather unlikely within the hematopoietic systems remains to be elucidated. the potential of Notch to expand HSCs in vitro could still be Collectively these gain- and loss-of-function studies clearly important in the exploration of specific therapeutic goals. demonstrate that Notch1 signaling is necessary and sufficient for T cell lineage commitment. Notch and T cell fate specification Two models have been proposed to explain how Notch The best-established role for Notch signaling in the hemato- regulates T cell lineage commitment. The first model is again poietic system is the essential function of Notch1 in T cell adapted from the function of Notch in influencing binary cell lineage commitment. Inducible inactivation of the Notch1 fate decisions from a bi-potential precursor, such as described gene in BM progenitors results in a block in thymic T cell previously for the development of mechanosensory bristles development at or before the earliest intrathymic precursor (Fig. 1). According to this model, Notch1 would regulate the stage. As a consequence of the Notch1 deficiency,immature B commitment of a bi-potential T/B precursor (Fig. 3). In the cells develop in the thymus from incoming BM progenitors absence of Notch1 signaling, bi-potential lymphoid progeni- suggesting that Notch1 instructs an early lymphoid progenitor tors are not instructed towards the Tcell lineage and thus adopt to adopt a T versus B cell fate.(56,57) Similar results have been a B cell fate in the thymus by default. As this has not been obtained by inducible inactivation of the CSL gene in BM demonstrated at the single cell level, an alternative scenario in progenitors.(58) These nearly identical phenotypes, coupled which cells entering the thymus are not bi- or (multi-) potent with the fact that CSL mediates signaling of all Notch receptors but represent a mixed population of progenitors with separ- strongly suggest that Tcell lineage commitment is mediated by ate lineage potential cannot be formally excluded. If this Notch1/CSL-dependent signaling in a non-redundant manner were the case, Notch1 signaling would instruct an already T despite expression of other Notch receptors in BM and/or lineage committed early lymphoid progenitor to further thymic progenitors.(34) In agreement with these results, no T differentiate towards the T cell lineage and simultaneously cell phenotype is observed after inducible inactivation of suppress the development of a separate B cell precursor. Notch2 in the hematopoietic system,(59) and to date no Despite this caveat, the simplest explanation is that Notch1 hematopoietic phenotypes have been reported for Notch3(60) regulates the fate of a bi-potential lymphoid progenitor cell or Notch4(61) gene targeted mice suggesting that T cell fate entering the thymus. specification is a non-redundant function of Notch1. In addition, these reciprocal gain- and loss-of-function The role of Notch signaling in specifying T cell fate studies support the concept that B cell and Tcell development has also been deduced from reciprocal gain-of-function must be compartmentalized, with B cell development occur- studies. First, transduction of BM progenitors with a retrovirus ring in the BM while the vast majority of T cell development containing a ligand-independent dominant active form of the takes place in the thymus. In order to allow B cell development Notch1 receptor (NIC), led to ectopic Tcell development in the in the BM, Notch signaling must be absent or negatively BM concomitant with a block in B cell development.(62) This regulated, despite the fact that some Notch receptors and particular phenotype is not intrinsic to the cytoplasmic domain ligands are expressed on BM progenitors and stroma.(34) One of Notch1 since similar results have been obtained with the possible explanation for this paradox is that hematopoietic cytoplasmic domain of Notch2(63) or Notch4.(64) Similarly, progenitors downregulate Notch receptors as soon as they retroviral transduction of BM progenitors with the Notch are engaged towards the B cell lineage. Evidence supporting ligands Delta1 or Delta 4 also results in ectopic T cell such a possibility stems from studies on the B lineage development and a simultaneous block in B cell develop- commitment factor Pax5, which represses transcription of ment.(65–67) This phenotype could be explained by continuous the Notch1 gene in B cell progenitors. Indeed, forced Pax5 activation of endogenous Notch receptors expressed on expression promotes B cell development at the expense of T hematopoietic progenitor cells within the BM compartment. cell development while all other blood lineages develop Another set of experiments supporting the essential role of normally.(72,73) An alternative hypothesis is that induction of Notch signaling for Tcell fate specification is based on the use the Tcell lineage commitment program would be restricted to a of negative modulators of the Notch pathway such as particular family of Notch ligands. In this scenario, only Delta-

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mediated and not Jagged-mediated Notch signaling would be ment. While Db to Jb rearrangement appears to be normal, able to induce T cell fate specification. Normal B cell rearrangement of Vb gene segments to DJb is perturbed.(79) development would occur as long as Delta expression is Decreased amounts of Vb-germline transcripts have been restricted to the thymus or to areas within the BM where B cell observed in Notch1-deficient progenitors suggesting that the development does not take place. Such a model is supported chromatin is not accessible to the recombination machin- by ectopic expression studies for Delta1 and Delta4 (see ery.(81) Thus Notch signaling may control chromatin accessi- above), in which T cell development is induced in the BM bility of Vb genes in the TCRb locus. The absence of Notch1 compartment concomitantly with a block in B cell develop- in immature ab lineage thymocytes results in an additional ment. To date such ectopic expression experiments using phenotype. Under normal conditions, those ab lineage pre-T ligands from the Jagged family have not been reported. cells that cannot form a functional pre-TCR complex are Further evidence that Delta and Jagged ligands elicit eliminated by apoptosis. However Notch1-deficient pre-Tcells qualitatively different Notch signals and thereby drive hema- lacking an in-frame TCRb chain accumulate in the thymus. topoietic progenitors into different developmental programs is Thus Notch1 appears to have two functions within the ab derived from in vitro studies using stromal cells expressing lineage, the first of which is linked to the successful different Notch ligands. While Delta1- or Delta4-expressing rearrangement of TCRb while the second involves the stroma can drive HSCs into the Tcell lineage, HSCs grown on elimination of thymocytes that fail to assemble a functional Jagged-expressing stromal cells develop into B cells.(74–76) In pre-TCR complex. A similar phenotype is observed in mice in agreement with these in vitro studies, Jagged2 gene targeted which the CSL gene is inactivated in immature thymocytes mice have apparently normal ab Tcell development and only a suggesting that Notch1-dependent functions within the ab minor decrease in gd T cells,(77) while inducible inactivation of lineage are CSL dependent.(80) Jagged1 has no effect on T cell development.(55) Collectively The final intrathymic cell fate decision is made by ab Tcells these observations would appear to exclude an important as CD4þCD8þ (DP) thymocytes must choose to adopt either a physiological role for Jagged family members during T cell CD4þ T helper- or a CD8þ cytotoxic-T cell fate. Several gain- development, thus raising the question of which of the Delta of-function studies have linked Notch signaling to this lineage ligands is involved. Surprisingly, T cell development in mice decision. Survival, maturation and influencing binary cell fate with an inducible inactivation of Delta1 was not perturbed(76) decisions were among the suggested roles for Notch signaling suggesting that either Delta4 must be the crucial ligand or that at this particular stage of development.(26,82–85) Delta1 and Delta4 can compensate for each other during Tcell However, since constitutive Notch signaling within the thymus lineage commitment. Further gene targeting studies are causes T cell leukemias, the results were very difficult to needed to address this issue. interpret. In contrast, conditional inactivation of either Notch1(86) or the CSL(80) gene in DP thymocytes does not Notch and intra-thymic T cell development result in any detectable skewing towards either the CD4 or Following commitment to the T cell lineage, immature pro-T CD8 lineage clearly indicating that Notch signaling is cells face two more lineage decisions. The first choice is dispensable for this cell fate decision. between the ab and gd Tcell fates while a further decision has In this review, we will not discuss the role of Notch during to be made within the ab Tcell lineage to either differentiate into peripheral T cell function. The interested reader is referred to CD4þ helper T cells or CD8þ cytotoxic T cells. several recently published reviews on this subject.(87–90) The original suggestion that Notch signaling influences the ab versus gd Tcell fate decision(78) has been challenged by two Notch and B cell development conditional loss-of-function approaches where either Notch1 While Notch1 is essential for T cell lineage specification, or CSL was inactivated in immature thymocyte progenitors. Notch2 is important during B cell development. The Notch Loss of Notch1 in developing thymocytes had no effect on gd T receptor that is predominantly expressed in B cells is Notch2, cell numbers or phenotype,(79) whereas loss of CSL resulted in while Notch1 and Notch3 are expressed in thymocytes and a moderate 2-fold increase in gd T cell numbers and an Notch4 is barely detectable in .(59,91) Expression accelerated turn-over rate as measured by BrdU incorpora- levels of Notch2 increase with B cell maturation and are tion.(80) While suggestive that Notch signaling may influence highest in splenic B cells suggesting a role for Notch signaling the rate of production of gd T cells, these data appear to in peripheral B cell development and/or function.(59) Indeed, exclude a critical role for Notch in ab versus gd T cell lineage conditional inactivation of the Notch2 gene leads to loss of a commitment. particular splenic B cell subset located on the margin of the B Although Notch1 deficiency in immature thymocyte pro- cell follicle at the blood–lymphoid interface, known as marginal genitors does not influence gd T cell development, it does zone B (MZB) cells.(59) MZB cells respond to blood-borne viral result in a partial block in ab T cell development. Notch1- and bacterial agents. Their rapid activation and differentiation deficient thymocytes are defective in TCRb gene rearrange- into -secreting plasma cells helps to bridge the gap

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between innate and adaptive immunity, the latter of which is ligand pair.Whether another specific receptor–ligand pair also mainly effected by follicular B cells (FoB).(92,93) Loss of MZB exists for Tcell fate specification will certainly be addressed in cells is also observed in mice in which the CSL gene is the near future. inactivated in B cells demonstrating that Notch2 signals via CSL to specify the MZB cell fate.(94) The development of Notch and neoplasia immature B cells in the BM compartment of both the Notch2- The first linking of Notch with Tcell neoplasia was made in the and the CSL conditional gene-targeted mice appears to be late 1980s(95) and early 1990s(96) with the cloning and normal suggesting that Notch signaling is dispensable for sequencing of a t (7;9) chromosomal translocation in a small early B cell development.(59,94) Loss of MZB cells in conditional subset of patients suffering from T cell acute lymphoblastic gene-targeted CSL mice is accompanied by a moderate leukemia (T-ALL). This chromosomal translocation juxtaposes increase in splenic FoB cells. A reciprocal splenic phenotype the C-terminal region of EGFrepeat 34 of the human NOTCH1 has been observed in MINT gene-targeted mice.(33) MINT is gene to the TCRb enhancer, which leads to the aberrant a negative modulator of Notch signaling, which is more expression of a truncated, dominant active and ligand- abundantly expressed in FoB cells compared to MZB cells. independent form of Notch1. This truncated Notch1 gene is MINT-deficient mice show an increase in MZB cell numbers also known as Notch1-IC and has been named TAN1 for with a concomitant reduction of FoB cells. These reciprocal translocation-associated Notch homolog.(96) The causative phenotypes have led to the suggestion that Notch signaling role of this truncated protein in disease development has been influences the commitment of a bi-potential splenic B cell confirmed in mouse models, as mice in which hematopoietic progenitor that has to choose between the MZB and FoB cell BM progenitors express TAN1 proteins develop T cell lineages (Fig. 3). To date it is unclear whether MZB cells and leukemia.(97) Similarly, constitutive expression of Notch1- FoB cells are derived from a common progenitor since a IC(26,82) or Notch3-IC(98) in immature thymocytes leads to skew towards FoB cells in the absence of MZB cells has not the development of T cell leukemia suggesting that the been observed in Notch2 conditional gene-targeted mice.(59) oncogenic potential of the Notch pathway is not restricted to Further investigation is required to clarify this discrepancy. Notch1. Furthermore T cell malignancies such as lymphopro- Most of our knowledge concerning Notch signaling and liferative disease(66) and Tcell leukemia can also be induced by lymphocyte development is derived from conditional gene- constitutive expression of the Delta4 ligand in BM progeni- targeted mice for Notch receptors (Notch1 and Notch2) or the tors.(65) Athough all these aberrant gain-of-function studies CSL transcription factor. Much less is known about the show that Notch can indeed be oncogenic, to date the physiological roles of Notch ligands. Whether ligand-receptor hematopoietic oncogenic potential seems to be restricted to specificity exists in vivo or whether different ligands elicit T cell leukemias since no myeloid leukemias have been qualitatively different responses when binding to the same reported. This suggests that Notch signaling on its own cannot Notch receptor is currently unclear. In this context, it is induce tumorigenesis but that Notch must cooperate with a T interesting to note that conditional inactivation of Delta1 in cell-specific signal in order to cause these Tcell malignancies. the hematopoietic system (using the interferon inducible Mx- Indeed Notch-IC- expressing BM progenitors derived from Cre system) resulted in the selective loss of MZB cells in the either Rag2/ or Lcp2/ (SLP76-deficient) mice do not spleen while Tcell development appeared to be normal.(76) As develop Tcell leukemias.(99) Both of these gene-targeted mice mentioned above, identical MZB cell phenotypes have been exhibit a developmental block during early Tcell differentiation observed in conditional gene-targeted mice for Notch2 and because these genes are required for pre-TCR signaling, an CSL indicating that Delta1-mediated Notch2/CSL signaling essential component of ab T cell development. In contrast, specifies MZB cell lineage commitment in a non-redundant expression of a transgenic TCR in Rag2-deficient mice fashion in vivo. However it is unclear whether Notch signaling restored the oncogenic potential of Notch-IC strongly suggest- occurs between homologous or heterologous cell types in ing that Notch-IC must cooperate with signals either mediated order to specify this particular B cell population. This question by the pre-TCR or the TCR itself to render T cells tumori- was addressed by B-cell-specific inactivation of Delta1 using genic.(99) The molecular details of Notch-induced tumorigen- the CD19-Cre system.(76) In this case, MZB cells were not lost esis are still not well understood and need further investigation. suggesting that Delta1–Notch2-mediated signaling must Experimentally Notch1 can collaborate with c-Myc,(100) E2A- occur between heterologous cells. Dendritic cells (DCs) were PBX1(101) and Ikaros.(102) Notch3 downregulates tumor suggested to mediate Notch2 signaling on B cell progenitors suppressor E2A activity(103) and activates protein kinase C based on the fact that DCs expressing Delta1 are found in which leads to activation of the NF-kB pathway.(104) close proximity to MZB cells at the margins of B cell follicles.(33) Although, in mouse models, the association of Notch and T However, this interpretation awaits further experimental cell leukemia has been widely demonstrated, the rare confirmation. In summary, these experiments demonstrate frequency of the t(7,9) translocation in humans (less than 1% that MZB cells are specified by a unique Notch receptor– of all T-ALL patients) questions the clinical importance of these

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findings. However recent studies by Aster and colleagues known. Nevertheless these data push Notch from being show that NOTCH signaling is one of the key players in all T- marginally involved in T-ALL to being one of the major players, ALL subtypes.(105) This group screened and identified a and highlight it as a potential therapeutic target for T cell number of T-ALL cell lines that undergo G0/G1 arrest when malignancies. cultured in the presence of g-secretase inhibitors (which inhibit cleavage of Notch receptors upon ligand-mediated activation). Subsequent sequencing of the NOTCH1 gene in these cell Concluding remarks lines identified two domains that were frequently mutated, one Notch signaling controls multiple cell fate decisions and of which is the extracellular heterodimerization domain (HD) differentiation processes during fetal hematopoiesis and adult and the other is the PEST domain, which is involved in lymphocyte development. Many of the crucial paradigms of regulating turnover of the protein. Analysis of 96 pediatric Notch function such as maintenance of undifferentiated primary T-ALL tumors revealed that 55% of these samples had progenitors or the control of binary cell fate decisions have at least one mutation in the HD or the PEST domain, with been established in invertebrates (Fig. 1). By analogy,many of approximately 20% of tumors having a mutation in both the lineage specification events controlled by Notch in the domains. Importantly transcriptional reporter assays con- mammalian hematopoietic system have been interpreted as firmed that NOTCH1 mutations within these tumor samples binary cell fate decisions in analogy to the concept originally correlated with increased Notch activity, which could be shown in Drosophila. For example, in the embryo, Notch suppressed by g-secretase inhibitors. Mutations in the HD signaling in hemangioblasts would specify hematopoietic domain lead to a 3- to 9-fold increase and mutations in the versus endothelial cell fates. In the adult, further Notch- PEST domain to an approximately 2-fold increase in Notch mediated lineage decisions such as T versus B, ab T versus gd activity. However mutations in cis (in the HD and the PEST T and MZB versus FoB could be viewed as binary cell fate domain simultaneously) resulted in a 20- to 40-fold increase in decisions (Fig. 3). However,an alternative interpretation is that Notch activity showing a striking synergistic effect. How these Notch signaling promotes previously specified differentiation point mutations in cis lead to synergistic Notch-mediated processes as has been shown for ab Tcell development or the transcriptional activation is mechanistically currently un- skin.

Figure 4. Chromosomal translocations and mutations within the human NOTCH1 gene cause T cell malignancies. A: The t(7;9) chromosomal translocation in human Tcell acute lymphoblastic leukemia (T-ALL) patients is characterized by juxtaposition of the 30 portion of the human NOTCH1 gene into the T cell receptor b (TCRb) locus. This leads to expression of truncated NOTCH1 transcripts and consequent production of dominant active, ligand-independent forms of the NOTCH1 receptor causing T-ALL. This rare event occurs in less than 1% of all T-ALLpatients. B: Schematic diagram of the full-length human NOTCH1 protein. Indicated are ‘hot spots’ of mutations found in more than 50% of T-ALL patients(105) HD, heterodimerization domain; P,Pest domain.

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Another commonly used analogy derived from inverte- 14. Lowell S, Jones P, Le Roux I, Dunne J, Watt FM. 2000. Stimulation of brates is the ‘gate-keeper’ function of Notch (maintenance of human epidermal differentiation by delta-notch signalling at the boundaries of stem-cell clusters. Curr Biol 10:491–500. the undifferentiated state) that has been postulated for 15. Rangarajan A, Talora C, Okuyama R, Nicolas M, Mammucari C, et al. hematopoietic stem cells or BM progenitor cells. Although 2001. Notch signaling is a direct determinant of keratinocyte growth experimentally Notch signaling has been partially associated arrest and entry into differentiation. Embo J 20:3427–3436. 16. Parks AL, Klueg KM, Stout JR, Muskavitch MA. 2000. Ligand with preventing BM progenitors from differentiation, to date we endocytosis drives receptor dissociation and activation in the Notch have no clearcut genetic evidence to support this notion. pathway. Development 127:1373–1385. However, it has recently become clear that aberrant Notch 17. Kao H, Ordentlich P, Koyano-Nakagawa N, Tang Z, Downes M, et al. 1998. A histone deacetylase corepressor complex regulates the Notch signaling in humans due to activating mutations in the Notch1 signal. Genes Dev 15:2269–2277. receptor plays a key role in the development of T-ALL (Fig. 4). 18. Hsieh JJ, Zhou S, Chen L, Young DB, Hayward SD. 1999. CIR, a Questions concerning specific Notch target genes, mechan- corepressor linking the DNA binding factor CBF1 to the histone. Proc Natl Acad Sci USA 96:23–28. istic insights into activating Notch mutations, ligand–receptor 19. Zhou S, Fujimuro M, Hsieh J, Chen L, Miyamoto A, et al. 2000. SKIP, a specificity, the physiological function of different Notch CBF1-associated protein, interacts with the ankyrin repeat domain CIR, modifiers and cross talk between Notch and other signaling a corepressor linking the DNA binding factor CBF1 to the histone. Mol Cell Biol 20:2400–2410. pathways remain to be investigated. Answers to such 20. Jeffries S, Robbins DJ, Capobianco AJ. 2002. Characterization of a questions will surely expand our limited understanding of cell high-molecular-weight Notch complex in the nucleus. Mol Cell Biol fate decisions and may ultimately lead to new strategies of how 11:3927–3941. 21. Wu L, Aster JC, Blacklow SC, Lake R, Artavanis-Tsakonas S, et al. to fight against cancer. 2000. MAML1, a human homologue of Drosophila mastermind, is a transcriptional. Nat Genet 4:484–489. 22. Fryer CJ, Lamar E, Turbachova I, Kintner C, Jones KA. 2002. Acknowledgments Mastermind mediates chromatin-specific transcription and turnover of We thank Pierre Dubied for preparation of the figures, and the Notch enhancer complex. Genes Dev 16:1397–1411. apologize to those whose work was not cited due to space 23. Davis RL, Turner DL. 2001. Vertebrate hairy and Enhancer of split related proteins: transcriptional repressors regulating cellular differ- limitations. entiation and embryonic patterning. Oncogene 20:8342–8357. 24. Iso T, Kedes L, Hamamori Y. 2003. HES and HERP families: multiple effectors of the Notch signaling pathway. J Cell Physiol 194:237– References 255. 1. Morgan TH. 1917. The theory of the gene. Am Nat 51:513–544. 25. Krebs LT, Deftos ML, Bevan MJ, Gridley T. 2001. The Nrarp gene 2. Wharton KA, Johansen KM, Xu T, Artavanis-Tsakonas S. 1985. encodes an ankyrin-repeat protein that is transcriptionally regulated by Nucleotide sequence from the neurogenic locus notch implies a gene the notch signaling pathway. Dev Biol 238:110–119. product that shares with proteins containing EGF-like 26. Deftos ML, Huang E, Ojala EW, Forbush KA, Bevan MJ. 2000. Notch1 repeats. Cell 43:567–581. signaling promotes the maturation of CD4 and CD8 SP thymocytes. 3. Kidd S, Kelley MR, Young MW. 1986. Sequence of the notch locus of Immunity 13:73–84. Drosophila melanogaster: relationship of the encoded protein to 27. Reizis B, Leder P. 2002. Direct induction of T lymphocyte-specific gene mammalian clotting and growth factors. Mol Cell Biol 6:3094–3108. expression by the mammalian Notch signaling pathway. Genes De 4. Artavanis-Tsakonas S, Rand MD, Lake RJ. 1999. Notch signaling: cell 16:295–300. fate control and signal integration in development. Science 284: 28. Haltiwanger RS, Stanley P. 2002. Modulation of receptor signaling by 770–776. glycosylation: fringe is an O-fucose-beta1,3-N-acetylglucosaminyl- 5. Artavanis-Tsakonas S, Delidakis C, Fehon RG. 1991. The Notch locus transferase. Biochim Biophys Acta 1573:328–335. and the cell biology of neuroblast segregation. Annu Rev Cell Biol 29. Haines N, Irvine K. 2003. Glycosylation regulates Notch signalling. Nat 7:427–452. Rev Mol Cell Biol 4:786–797. 6. Furukawa T, Maruyama S, Kawaichi M, Honjo T. 1992. The Drosophila 30. Matsuno K, Diederich RJ, Go MJ, Blaumueller CM, Artavanis-Tsakonas homolog of the immunoglobulin recombination signal-binding protein S. 1995. Deltex acts as a positive regulator of Notch signaling regulates peripheral nervous system development. Cell 69:1191–1197. through interactions with the Notch ankyrin repeats. Development 7. Schweisguth F, Posakony JW. 1992. Suppressor of Hairless, the 121:2633–2644. Drosophila homolog of the mouse recombination signal-binding protein 31. Frise E, Knoblich JA, Younger-Shepherd S, Jan LY, Jan YN. 1996. The gene, controls sensory organ cell fates. Cell 69:1199–1212. Drosophila Numb protein inhibits signaling of the Notch receptor during 8. Poulson DF. 1937. Chromosomal deficiencies and the embryonic cell-cell interaction in sensory organ lineage. Proc Natl Acad Sci USA development of Drosophila Melanogaster. PNAS 23:133–137. 93:11925–11932. 9. Knust E, Campos-Ortega JA. 1989. The molecular genetics of early 32. Yun TJ, Bevan MJ. 2003. Notch-regulated ankyrin-repeat protein in Drosophila melanogaster. Bioessays 11:95–100. inhibits Notch1 signaling: multiple Notch1 signaling pathways involved 10. Coffman CR, Skoglund P, Harris WA, Kintner CR. 1993. Expression of in T cell development. J Immunol 170:5834–5841. an extracellular deletion of Xotch diverts cell fate in Xenopus embryos. 33. Kuroda K, Han H, Tani S, Tanigaki K, Tun T, et al. 2003. Regulation of Cell 73:659–671. marginal zone B cell development by MINT, a suppressor of Notch/ 11. Chitnis A, Henrique D, Lewis J, Ish-Horowicz D, Kintner C. 1995. RBP-J signaling pathway. Immunity 18:301–312. Primary neurogenesis in Xenopus embryos regulated by a homologue 34. Radtke F, Wilson A, Mancini SJ, MacDonald HR. 2004. Notch of the Drosophila neurogenic gene Delta. Nature 375:761–766. regulation of lymphocyte development and function. Nat Immunol 12. Nye JS, Kopan R, Axel R. 1994. An activated Notch suppresses 5:247–253. neurogenesis and but not gliogenesis in mammalian cells. 35. Kumano K, Chiba S, Kunisato A, Sata M, Saito T, et al. 2003. Notch1 but Development 120:2421–2430. not Notch2 is essential for generating hematopoietic stem cells from 13. Jan YN, Jan LY. 1994. Genetic control of cell fate specification endothelial cells. Immunity 18:699–711. in Drosophila peripheral nervous system. Annu Rev Genet 28: 36. Godin I, Cumano A. 2002. The hare and the tortoise: an embryonic 373–393. haematopoietic race. Nat Rev Immunol 2:593–604.

1126 BioEssays 27.11 Review articles

37. Pardanaud L, Yassine F, Dieterlen-Lievre F. 1989. Relationship 59. Saito T, Chiba S, Ichikawa M, Kunisato A, Asai T, et al. 2003. between vasculogenesis, angiogenesis and haemopoiesis during Notch2 is preferentially expressed in mature B cells and indispens- avian ontogeny. Development 105:473–485. able for marginal zone B lineage development. Immunity 18:675– 38. Cho K, Kennedy M, Kazarov A, Papadimitriou JC, Keller G. 1998. A 685. common precursor for hematopoietic and endothelial cells. Develop- 60. Krebs L, Xue Y, Norton C, Sundberg J, Beatus P, et al. 2003. ment 125:725–732. Characterization of Notch3-deficient mice: Normal embryonic devel- 39. de Bruijn MF, Ma X, Robin C, Ottersbach K, Sanchez MJ, et al. 2002. opment. Genesis 3:139–143. Hematopoietic stem cells localize to the endothelial cell layer in the 61. Krebs LT, Xue Y, Norton CR, Shutter JR, Maguire M, et al. 2000. Notch midgestation mouse aorta. Immunity 16:673–683. signaling is essential for vascular in mice. Genes Dev 40. Robert-Moreno A, Espinosa L, de la Pompa JL, Bigas A. 2005. 14:1343–1352. RBPj{kappa}-dependent Notch function regulates Gata2 and is 62. Pui JC, Allman D, Xu L, DeRocco S, Karnell FG, et al. 1999. Notch1 essential for the formation of intra-embryonic hematopoietic cells. expression in early lymphopoiesis influences B versus T lineage Development 132:1117–1126. determination. Immunity 11:299–308. 41. Tsai FY, Keller G, Kuo FC, Weiss M, Chen J, et al. 1994. An early 63. Witt CM, Hurez V, Swindle CS, Hamada Y, Klug CA. 2003. Activated haematopoietic defect in mice lacking the transcription factor GATA-2. Notch2 potentiates CD8 lineage maturation and promotes the selective Nature 371:221–226. development of B1 B cells. Mol Cell Biol 23:8637–8650. 42. Shawber CJ, Kitajewski J. 2004. Notch function in the vasculature: 64. Vercauteren SM, Sutherland HJ. 2004. Constitutively active Notch4 insights from zebrafish, mouse and man. Bioessays 26:225–234. promotes early human hematopoietic progenitor cell maintenance 43. Stier S, Cheng T, Dombkowski D, Carlesso N, Scadden DT. 2002. while inhibiting differentiation and causes lymphoid abnormalities in Notch1 activation increases hematopoietic stem cell self-renewal vivo. Blood 104:2315–2322. in vivo and favors lymphoid over myeloid lineage outcome. Blood 65. Yan XQ, Sarmiento U, Sun Y, Huang G, Guo J, et al. 2001. A novel 99:2369–2378. Notch ligand, Dll4, induces T-cell leukemia/ when over- 44. Varnum-Finney B, Xu L, Brashem-Stein C, Nourigat C, Flowers D, expressed in mice by retroviral-mediated gene transfer. Blood et al. 2000. Pluripotent, -dependent, hematopoietic stem 98:3793–3799. cells are immortalized by constitutive Notch1 signaling. Nat Med 6: 66. Dorsch M, Zheng G, Yowe D, Rao P, Wang Y, et al. 2002. Ectopic 1278–1281. expression of Delta4 impairs hematopoietic development and leads to 45. Karanu FN, Murdoch B, Gallacher L, Wu DM, Koremoto M, et al. 2000. lymphoproliferative disease. Blood 100:2046–2055. The notch ligand jagged-1 represents a novel growth factor of human 67. de La Coste A, Six E, Fazilleau N, Mascarell L, Legrand N, et al. hematopoietic stem cells. J Exp Med 192:1365–1372. 2005. In vivo and in absence of a thymus, the enforced ex- 46. Karanu FN, Yuefei L, Gallacher L, Sakano S, Bhatia M. 2003. Differential pression of the notch ligands delta-1 or delta-4 promotes T cell response of primitive human CD34- and CD34þ hematopoietic cells to development with specific unique effects. J Immunol 174:2730– the Notch ligand Jagged-1. Leukemia 17:1366–1374. 2737. 47. Varnum-Finney B, Purton LE, Yu M, Brashem-Stein C, Flowers D, et al. 68. Koch U, Lacombe TA, Holland D, Bowman JL, Cohen BL, et al. 2001. 1998. The Notch ligand, Jagged-1, influences the development of Subversion of the T/B lineage decision in the thymus by lunatic fringe- primitive hematopoietic precursor cells. Blood 91:4084–4091. mediated inhibition of Notch-1. Immunity 15:225–236. 48. Ohishi K, Varnum-Finney B, Bernstein ID. 2002. Delta-1 enhances 69. Izon DJ, Aster JC, He Y, Weng A, Karnell FG, et al. 2002. Deltex1 marrow and thymus repopulating ability of human CD34(þ)CD38() redirects lymphoid progenitors to the B cell lineage by antagonizing cord blood cells. J Clin Invest 110:1165–1174. Notch1. Immunity 16:231–243. 49. Karanu FN, Murdoch B, Miyabayashi T, Ohno M, Koremoto M, et al. 70. Maillard I, Weng AP, Carpenter AC, Rodriguez CG, Sai H, et al. 2004. 2001. Human homologues of Delta-1 and Delta-4 function as mitogenic Mastermind critically regulates Notch-mediated lymphoid cell fate regulators of primitive human hematopoietic cells. Blood 97:1960–1967. decisions. Blood 104:1696–1702. 50. Varnum-Finney B, Brashem-Stein C, Bernstein ID. 2003. Combined 71. French MB, Koch U, Shaye RE, McGill MA, Dho SE, et al. 2002. effects of Notch signaling and induce a multiple log increase Transgenic expression of numb inhibits notch signaling in immature in precursors with lymphoid and myeloid reconstituting ability. Blood thymocytes but does not alter T cell fate specification. J Immunol 101:1784–1789. 168:3173–3180. 51. Carlesso N, Aster JC, Sklar J, Scadden DT. 1999. Notch1-induced 72. Souabni A, Cobaleda C, Schebesta M, Busslinger M. 2002. Pax5 delay of human hematopoietic progenitor cell differentiation is promotes B lymphopoiesis and blocks T cell development by associated with altered cell cycle kinetics. Blood 93:838–848. repressing Notch1. Immunity 17:781–793. 52. Goodell MA, Brose K, Paradis G, Conner AS, Mulligan RC. 1996. 73. Cotta CV, Zhang Z, Kim HG, Klug CA. 2003. Pax5 determines B-versus Isolation and functional properties of murine hematopoietic stem cells T-cell fate and does not block early myeloid-lineage development. that are replicating in vivo. J Exp Med 183:1797–1806. Blood 101:4342–4346. 53. Duncan AW, Rattis FM, DiMascio LN, Congdon KL, Pazianos G, et al. 74. Jaleco AC, Neves H, Hooijberg E, Gameiro P, Clode N, et al. 2001. 2005. Integration of Notch and Wnt signaling in hematopoietic stem cell Differential effects of Notch ligands Delta-1 and Jagged-1 in human maintenance. Nat Immunol 6:314–322. lymphoid differentiation. J Exp Med 194:991–1002. 54. Calvi L, Adams G, Weibrecht K, Weber J, Olson D, et al. 2003. 75. Schmitt TM, Zuniga-Pflucker JC. 2002. Induction of T cell development Osteoblastic cells regulate the haematopoietic stem cell niche. Nature from hematopoietic progenitor cells by delta-like-1 in vitro. Immunity 425:841–846. 17:749–756. 55. Mancini SJ, Mantei N, Dumortier A, Suter U, Macdonald HR, et al. 2005. 76. Hozumi K, Negishi N, Suzuki D, Abe N, Sotomaru Y, et al. 2004. Delta- Jagged1-dependent Notch signaling is dispensable for hematopoietic like 1 is necessary for the generation of marginal zone B cells but not T stem cell self-renewal and differentiation. Blood 105:2340–2342. cells in vivo. Nat Immunol 5:638–644. 56. Radtke F, Wilson A, Stark G, Bauer M, van Meerwijk J, et al. 1999. 77. Jiang R, Lan Y, Chapman HD, Shawber C, Norton CR, et al. 1998. Deficient T cell fate specification in mice with an induced inactivation of Defects in limb, craniofacial, and thymic development in Jagged2 Notch1. Immunity 10:547–558. mutant mice. Genes Dev 12:1046–1057. 57. Wilson A, MacDonald HR, Radtke F. 2001. -deficient common 78. Washburn T, Schweighoffer E, Gridley T, Chang D, Fowlkes BJ, et al. lymphoid precursors adopt a B cell fate in the thymus. J Exp Med 1997. Notch activity influences the alphabeta versus gammadelta T cell 194:1003–1012. lineage decision. Cell 88:833–843. 58. Han H, Tanigaki K, Yamamoto N, Kuroda K, Yoshimoto M, et al. 2002. 79. Wolfer A, Wilson A, Nemir M, MacDonald HR, Radtke F. 2002. Inducible gene knockout of transcription factor recombination signal Inactivation of Notch1 impairs VDJbeta rearrangement and allows binding protein-J reveals its essential role in T versus B lineage pre-TCR-independent survival of early alpha beta Lineage Thymocytes. decision. Int Immunol 14:637–645. Immunity 16:869–879.

BioEssays 27.11 1127 Review articles

80. Tanigaki K, Tsuji M, Yamamoto N, Han H, Tsukada J, et al. 2004. 94. Tanigaki K, Han H, Yamamoto N, Tashiro K, Ikegawa M, et al. 2002. Regulation of alphabeta/gammadelta T cell lineage commitment and Notch-RBP-J signaling is involved in of marginal peripheral T cell responses by Notch/RBP-J signaling. Immunity 20: zone B cells. Nat Immunol 3:443–450. 611–622. 95. Reynolds TC, Smith SD, Sklar J. 1987. Analysis of DNA surrounding 81. Hoflinger S, Kesavan K, Fuxa M, Hutter C, Heavey B, et al. the breakpoints of chromosomal translocations involving the beta T cell 2004. Analysis of Notch1 function by in vitro T cell differentiation receptor gene in human lymphoblastic neoplasms. Cell 50:107–117. of Pax5 mutant lymphoid progenitors. J Immunol 173:3935–3944. 96. Ellisen LW, Bird J, West DC, Soreng AL, Reynolds TC, et al. 1991. TAN- 82. Robey E, Chang D, Itano A, Cado D, Alexander H, et al. 1996. An 1, the human homolog of the Drosophila notch gene, is broken by activated form of Notch influences the choice between CD4 and CD8 T chromosomal translocations in T lymphoblastic neoplasms. Cell 66: cell lineages. Cell 87:483–492. 649–661. 83. Izon DJ, Punt JA, Xu L, Karnell FG, Allman D, et al. 2001. Notch1 97. Pear WS, Aster JC, Scott ML, Hasserjian RP, Soffer B, et al. 1996. regulates maturation of CD4þ and CD8þ thymocytes by modulating Exclusive development of T cell neoplasms in mice transplanted with TCR signal strength. Immunity 14:253–264. bone marrow expressing activated Notch alleles. J Exp Med 183: 84. Deftos ML, He YW, Ojala EW, Bevan MJ. 1998. Correlating notch 2283–2291. signaling with thymocyte maturation. Immunity 9:777–786. 98. Bellavia D, Campese AF, Alesse E, Vacca A, Felli MP, et al. 2000. 85. Fowlkes BJ, Robey EA. 2002. A reassessment of the effect of activated Constitutive activation of NF-kappaB and T-cell leukemia/lymphoma in Notch1 on CD4 and CD8 T cell development. J Immunol 169:1817– Notch3 transgenic mice. Embo J 19:3337–3348. 1821. 99. Allman D, Karnell FG, Punt JA, Bakkour S, Xu L, et al. 2001. Separation 86. Wolfer A, Bakker T, Wilson A, Nicolas M, Ioannidis V, et al. 2001. of Notch1 promoted lineage commitment and expansion/transformation Inactivation of Notch 1 in immature thymocytes does not perturb CD4 or in developing T cells. J Exp Med 194:99–106. CD8 T cell development. Nat Immunol 2:235–241. 100. Girard L, Hanna Z, Beaulieu N, Hoemann CD, Simard C, et al. 1996. 87. Maillard I, Fang T, Pear WS. 2005. Regulation of Lymphoid Develop- Frequent provirus insertional mutagenesis of Notch1 in thymomas of ment, Differentiation, and Function by the Notch Pathway. Annu Rev MMTVD/myc transgenic mice suggests a collaboration of c-myc and Immunol 23:945–974. Notch1 for oncogenesis. Genes Dev 10:1930–1944. 88. Tsukumo S, Yasutomo K. 2004. Notch governing mature T cell 101. Feldman BJ, Hampton T, Cleary ML. 2000. A carboxy-terminal deletion differentiation. J Immunol 173:7109–7113. mutant of Notch1 accelerates lymphoid oncogenesis in E2A-PBX1 89. Robey EA, Bluestone JA. 2004. Notch signaling in lymphocyte transgenic mice. Blood 96:1906–1913. development and function. Curr Opin Immunol 16:360–366. 102. Beverly LJ, Capobianco AJ. 2003. Perturbation of Ikaros isoform 90. Dallman MJ, Champion B, Lamb JR. 2003. Notch signalling in the selection by MLV integration is a cooperative event in Notch(IC)- peripheral . Novartis Found Symp 252:268–276; induced T cell leukemogenesis. Cancer Cell 3:551–564. discussion 276–268. 103. Talora C, Campese AF, Bellavia D, Pascucci M, Checquolo S, et al. 91. Huang EY, Gallegos AM, Richards SM, Lehar SM, Bevan MJ. 2003. 2003. Pre-TCR-triggered ERK signalling-dependent downregulation of Surface expression of Notch1 on thymocytes: correlation with the E2A activity. EMBO Rep 4:1067–1072. double-negative to double-positive transition. J Immunol 171:2296– 104. Felli MP, Vacca A, Calce A, Bellavia D, Campese AF, et al. 2005. PKC 2304. theta mediates pre-TCR signaling and contributes to Notch3-induced 92. Lopes-Carvalho T, Kearney JF. 2004. Development and selection of T-cell leukemia. Oncogene 24:992–1000. marginal zone B cells. Immunol Rev 197:192–205. 105. Weng AP, Ferrando AA, Lee W, Morris JP, Silverman LB, et al. 2004. 93. Pillai S, Cariappa A, Moran ST. 2005. Marginal zone B cells. Annu Rev Activating mutations of NOTCH1 in human T cell acute lymphoblastic Immunol 23:161–196. leukemia. Science 306:269–271.

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