Regulatory cross talks of bone cells, hematopoietic stem cells and the nervous system maintain hematopoiesis

Adult hematopoietic stem cells (HSC) continuously replenish the blood with immune and blood cells with a finite life span, from the bone marrow (BM) reservoir of immature and maturing leukocytes. Regulation of HSC migration and development is essential for their function and blood cell production. These diverse and multiple states require a tight regulation to efficiently address host defense and repair requirements. Numerous recent studies disclose a central role for bone related cells in regulation of HSC and hematopoiesis. During ontogeny HSC home and seed the fetal BM in the last gestation period when the bone is already ossified. Ossification involves bone forming osteoblast- and bone degrading osteoclast activity and is considered essential for the formation of BM cavities and hematopoiesis. This synchronized association implies the need for active bone cells and bone turnover for HSC regulation. Osteoblastic cells and SDF- 1+/nestin+ reticular adventitial and CAR cells are crucial for regulation of HSC lodgment, self-renewal and function. Bone resorbing osteoclasts regulate bone turnover and progenitor cell detachment and release from the BM. Sympathetic signals from the nervous system activated by circadian rhythms or stress conditions control both bone turnover and HSC migration and development. In this review we discuss pathways and mechanisms involved in this orchestrated regulatory network. A special focus is made on the pivotal role of the SDF-1/CXCR4 axis as a determinant of HSC fate. Inflammation, DNA damage, cytokine mobilization, microgravity and aging are discussed as specific physiologic and pathologic events entailing dysregulation of the tightly balanced Bone-Brain-Blood triad.

Quantifying hematopoietic stem and progenitor cell mobilization

Allogeneic donor blood cells and autologous peripheral blood leukocytes (PBL), obtained following -clinical mobilization procedures, are routinely used as a major source of hematopoietic stem and progenitor cells (HSPC) for transplantation protocols. It is, therefore, essential to evaluate and to quantify the extent by which the HSPC are mobilized and enriched in the circulation in correlation with their long-term hematopoietic reconstitution capacity. In this chapter, we describe quantitative methods that measure the number of mobilized HSPC according to specific criteria, as well as their functional properties in vitro and in vivo. The described assays are useful for assessment of progenitor cell mobilization as applied to both human and murine HSPC.

Insights into the mechanism of enhanced mobilization of hematopoietic progenitor cells and release of CXCL12 by a combination of AMD3100 and aminoglycoside-polyarginine conjugates

Mobilization of hematopoietic stem and progenitor cells (HSPCs) from the bone marrow to the peripheral blood is utilized in clinical HSPC transplantation protocols. Retention of HSPCs in the bone marrow is determined by relationships between the chemokine chemokine (C-X-C motif) ligand 12 (CXCL12) and its major receptor C-X-C chemokine receptor type 4 (CXCR4), and disruption of this retention by CXCR4 antagonists such as AMD3100 induces rapid HSPC mobilization. Here, we report that aminoglycoside-polyarginine conjugates (APACs) and N-α-acetyl-nona-D-arginine (r9) induce mobilization of white blood cells and, preferentially, immature hematopoietic progenitor cells (HPCs) in mice, similarly to AMD3100. Remarkably, administration of AMD3100 with each one of the APACs or r9 caused additional HPC mobilization. The mobilizing activity of APACs and r9 was accompanied by a significant elevation in plasma CXCL12 levels. To further understand how APACs, r9 and their combinations with AMD3100 compete with CXCL12 binding to CXCR4, as well with antibody against CXCR4 for CXCR4 binding, we have undertaken an approach combining experimental validation and docking to determine plausible binding modes for these ligands. On the basis of our biological and docking findings, and recently published NMR data, we suggest that combination of pairs of compounds such as APACs (or r9) with AMD3100 induces more efficient disruption of the CXCL12-CXCR4 interaction than AMD3100 alone, resulting in enhanced HPC mobilization.

CD45 regulates homing and engraftment of immature normal and leukemic human cells in transplanted immunodeficient mice

Bone marrow homing and engraftment by clinically transplanted hematopoietic stem and progenitor cells is a complex process that is not fully understood. We report that the pan-leukocyte CD45 phosphatase plays an essential role in trafficking and repopulation of the bone marrow by immature human CD34(+) cells and leukemic cells in transplanted nonobese diabetic severe combined immunodeficient mice. Inhibiting CD45 function by blocking antibodies or a CD45 inhibitor impaired the motility of both normal and leukemic human cells. Blocking CD45 inhibited homing and repopulation by immature human CD34(+) cells as well as homing of primary patient leukemic cells. In addition, CD45 inhibition negatively affected development of hematopoietic progenitors in vitro and their recovery in transplanted recipients in vivo, revealing the central role of CD45 in the regulation of hematopoiesis. Moreover, CD45 blockage induced a hyperadhesive phenotype in immature human progenitor cells as well as in murine leukocytes, leading to their defective adhesion interactions with endothelial cells. This phenotype was further manifested by the ability of CD45 blockage to prevent breakdown of adhesion interactions in the BM, which inhibited murine progenitor mobilization. The substantial effects of a direct CD45 inhibition point at its essential roles in cell trafficking, including murine progenitor cell mobilization and both normal immature and leukemic human hematopoietic cells as well as regulation of hematopoiesis and engraftment potential.

MT1-MMP and RECK: opposite and essential roles in hematopoietic stem and progenitor cell retention and migration

Migratory capacity is a fundamental property of hematopoietic stem and progenitor cells (HSPCs). This feature is employed in clinical mobilization of HSPCs to the circulation and constitutes the basis for modern bone marrow (BM) transplantation procedures which are routinely used to treat hematological malignancies. Therefore, characterization of new players in the complex process of HSPC motility in steady-state conditions as well as during stress situations is a major challenge. We report that while the metalloproteinase membrane type 1-metalloprotease (MT1-MMP) has an essential role in human HSPC trafficking during granulocyte colony-stimulating factor (G-CSF)-induced mobilization, its inhibitor reversion-inducing cysteine-rich protein with Kazal motifs (RECK) and the adhesion molecule CD44 are required for HSPC retention to the BM in steady-state conditions. The nervous system via Wnt signaling along with HGF/c-Met signaling and the complement cascade play a major role in regulating MT1-MMP increased activity, CD44 cleavage, and RECK-reduced expression during G-CSF-induced mobilization. This review will elaborate on the opposite roles of MT1-MMP and RECK in HSPC migration and retention and suggest targeting them in order to facilitate HSPC mobilization and engraftment upon BM transplantation in patients.

Rapid mobilization of hematopoietic progenitors by AMD3100 and catecholamines is mediated by CXCR4-dependent SDF-1 release from bone marrow stromal cells

Steady-state egress of hematopoietic progenitor cells can be rapidly amplified by mobilizing agents such as AMD3100, the mechanism, however, is poorly understood. We report that AMD3100 increased the homeostatic release of the chemokine stromal cell derived factor-1 (SDF-1) to the circulation in mice and non-human primates. Neutralizing antibodies against CXCR4 or SDF-1 inhibited both steady state and AMD3100-induced SDF-1 release and reduced egress of murine progenitor cells over mature leukocytes. Intra-bone injection of biotinylated SDF-1 also enhanced release of this chemokine and murine progenitor cell mobilization. AMD3100 directly induced SDF-1 release from CXCR4(+) human bone marrow osteoblasts and endothelial cells and activated uPA in a CXCR4/JNK-dependent manner. Additionally, ROS inhibition reduced AMD3100-induced SDF-1 release, activation of circulating uPA and mobilization of progenitor cells. Norepinephrine treatment, mimicking acute stress, rapidly increased SDF-1 release and progenitor cell mobilization, whereas β2-adrenergic antagonist inhibited both steady state and AMD3100-induced SDF-1 release and progenitor cell mobilization in mice. In conclusion, this study reveals that SDF-1 release from bone marrow stromal cells to the circulation emerges as a pivotal mechanism essential for steady-state egress and rapid mobilization of hematopoietic progenitor cells, but not mature leukocytes.

Neural regulation of bone, marrow, and the microenvironment

Bone marrow niches are specialized microenvironments comprising a heterogeneous population of cells that support and regulate hematopoietic stem and progenitor cells. Considerable advances made in the field of hematopoiesis reveal a cross talk between cells, cytokines and neurotransmitters of the hematopoietic, immune and skeletal systems. Dynamic modulation and regulation of stem cells and their niches in response to internal and external stimuli are essential for homeostasis, host defense and repair. This review presents evidence to substantiate stem cell regulation via the "brain-bone-blood triad" beginning at the embryonic stage and continuing to adulthood.

CXCL12 secretion by bone marrow stromal cells is dependent on cell contact and mediated by connexin-43 and connexin-45 gap junctions

The chemokine CXCL12 is essential for the function of hematopoietic stem and progenitor cells. Here we report that secretion of functional CXCL12 from human bone marrow stromal cells (BMSCs) was a cell contact-dependent event mediated by connexin-43 (Cx43) and Cx45 gap junctions. Inhibition of connexin gap junctions impaired the secretion of CXCL12 and homing of leukocytes to mouse bone marrow. Purified human CD34(+) progenitor cells did not adhere to noncontacting BMSCs, which led to a much smaller pool of immature cells. Calcium conduction activated signaling by cAMP-protein kinase A (PKA) and induced CXCL12 secretion mediated by the GTPase RalA. Cx43 and Cx45 additionally controlled Cxcl12 transcription by regulating the nuclear localization of the transcription factor Sp1. We suggest that BMSCs form a dynamic syncytium via connexin gap junctions that regulates CXC12 secretion and the homeostasis of hematopoietic stem cells.

Pathways implicated in stem cell migration: the SDF-1/CXCR4 axis

The hallmark of hematopoietic stem and progenitor cells (HSPCs) is their motility, which is essential for their function, such as recruitment upon demand. Stromal Derived Factor-1 (SDF-1, CXCL12) and its major receptor CXCR4 play major roles in stem cell motility and development. In vitro migration assays, implicating either gradients or cell surface-bound forms of SDF-1, are easy to perform and provide vital information regarding directional and random stem cell motility, which correlate with their repopulation potential in clinical and experimental transplantations. In vivo stem cell homing to the bone marrow, their retention, engraftment, and egress to the circulation, all involve SDF-1/CXCR4 interactions. Finally, other stem cell features such as stem cell survival and proliferation, are also dependent on the SDF-1/CXCR4 axis.

The brain-bone-blood triad: traffic lights for stem-cell homing and mobilization

Navigation of transplanted stem cells to their target organs is essential for clinical bone marrow reconstitution. Recent studies have established that hematopoietic stem cells (HSCs) dynamically change their features and location, shifting from quiescent and stationary cells anchored in the bone marrow to cycling and motile cells entering the circulation. These changes are driven by stress signals. Bidirectional migrations to and from the bone marrow are active processes that form the basis for HSC transplantation protocols. However, how and why HSCs enter and exit the bone marrow as part of host defense and repair is not fully understood. The development of functional, preclinical, immune-deficient NOD/SCID (non-obese diabetic-severe combined immunodeficiency) mice transplantation models has enabled the characterization of normal and leukemic human HSCs and investigation of their biology. Intensive research has revealed multiple tasks for the chemokine SDF-1 (stromal cell-derived factor-1, also known as CXCL12) in HSC interactions with the microenvironment, as well as the existence of overlapping mechanisms controlling stress-induced mobilization and enhanced HSC homing, sequential events of major physiological relevance. These processes entail dynamically interacting, multi-system aspects that link the bone marrow vasculature and stromal cells with the nervous and immune systems. Neural cues act as an external pacemaker to synchronize HSC migration and development to balance bone remodeling via circadian rhythms in order to address blood and immune cell production for the physiological needs of the body. Stress situations and clinical HSC mobilization accelerate leukocyte proliferation and bone turnover. This review presents the concept that HSC regulation by the brain-bone-blood triad via stress signals controls the bone marrow reservoir of immature and maturing leukocytes.

Blood-forming stem cells are nervous: direct and indirect regulation of immature human CD34+ cells by the nervous system

The nervous system regulates immunity through hormonal and neuronal routes as part of host defense and repair mechanism. Here, we review the emerging evidence for regulation of human hematopoietic stem and progenitor cells (HSPC) by the nervous system both directly and indirectly via their bone marrow (BM) niche-supporting stromal cells. Functional expression of several neurotransmitter receptors was demonstrated on HSPC, mainly on the more primitive CD34(+)/CD38(-/low) fraction. The myeloid cytokines, G-CSF and GM-CSF, dynamically upregulate neuronal receptor expression on human HSPC. This is followed by an increased response to neurotransmitters, leading to enhanced proliferation and motility of human CD34(+) progenitors, repopulation of the murine BM and their egress to the circulation. Importantly, recent observations showed rapid mobilization of human HSPC to high SDF-1 expressing ischemic tissues of stroke individuals followed by neoangiogenesis, neurological and functional recovery. Along with decreased levels of circulating immature CD34(+) cells and SDF-1 blood levels found in patients with early-stage Alzheimer's disease, these findings suggest a possible involvement of human HSPC in brain homeostasis and thus their potential clinical applications in neuropathology.

MT1-MMP and RECK are involved in human CD34+ progenitor cell retention, egress, and mobilization

The mechanisms governing hematopoietic progenitor cell mobilization are not fully understood. We report higher membrane type 1-MMP (MT1-MMP) and lower expression of the MT1-MMP inhibitor, reversion-inducing cysteine-rich protein with Kazal motifs (RECK), on isolated circulating human CD34+ progenitor cells compared with immature BM cells. The expression of MT1-MMP correlated with clinical mobilization of CD34+ cells in healthy donors and patients with lymphoid malignancies. Treatment with G-CSF further increased MT1-MMP and decreased RECK expression in human and murine hematopoietic cells in a PI3K/Akt-dependent manner, resulting in elevated MT1-MMP activity. Blocking MT1-MMP function by Abs or siRNAs impaired chemotaxis and homing of G-CSF-mobilized human CD34+ progenitors. The mobilization of immature and maturing human progenitors in chimeric NOD/SCID mice by G-CSF was inhibited by anti-MT1-MMP treatment, while RECK neutralization promoted motility and egress of BM CD34+ cells. BM c-kit+ cells from MT1-MMP-deficient mice also exhibited inferior chemotaxis, reduced homing and engraftment capacities, and impaired G-CSF-induced mobilization in murine chimeras. Membranal CD44 cleavage by MT1-MMP was enhanced following G-CSF treatment, reducing CD34+ cell adhesion. Accordingly, CD44-deficient mice had a higher frequency of circulating progenitors. Our results reveal that the motility, adhesion, homing, and mobilization of human hematopoietic progenitor cells are regulated in a cell-autonomous manner by dynamic and opposite changes in MT1-MMP and RECK expression.

The Wnt Antagonist Dickkopf-1 Mobilizes Vasculogenic Progenitor Cells via Activation of the Bone Marrow Endosteal Stem Cell Niche

Therapeutic mobilization of vasculogenic progenitor cells is a novel strategy to enhance neovascularization for tissue repair. Prototypical mobilizing agents such as granulocyte colony-stimulating factor mobilize vasculogenic progenitor cells from the bone marrow concomitantly with inflammatory cells. In the bone marrow, mobilization is regulated in the stem cell niche, in which endosteal cells such as osteoblasts and osteoclasts play a key role. Because Wnt signaling regulates endosteal cells, we examined whether the Wnt signaling antagonist Dickkopf (Dkk)-1 is involved in the mobilization of vasculogenic progenitor cells. Using TOP-GAL transgenic mice to determine activation of β-catenin, we demonstrate that Dkk-1 regulates endosteal cells in the bone marrow stem cell niche and subsequently mobilizes vasculogenic and hematopoietic progenitors cells without concomitant mobilization of inflammatory neutrophils. The mobilization of vasculogenic progenitors required the presence of functionally active osteoclasts, as demonstrated in PTPε-deficient mice with defective osteoclast function. Mechanistically, Dkk-1 induced the osteoclast differentiation factor RANKL, which subsequently stimulated the release of the major bone-resorbing protease cathepsin K. Eventually, the Dkk-1–induced mobilization of bone marrow–derived vasculogenic progenitors enhanced neovascularization in Matrigel plugs. Thus, these data show that Dkk-1 is a mobilizer of vasculogenic progenitors but not of inflammatory cells, which could be of great clinical importance to enhance regenerative cell therapy.

CD45 regulates retention, motility, and numbers of hematopoietic progenitors, and affects osteoclast remodeling of metaphyseal trabecules

The CD45 phosphatase is uniquely expressed by all leukocytes, but its role in regulating hematopoietic progenitors is poorly understood. We show that enhanced CD45 expression on bone marrow (BM) leukocytes correlates with increased cell motility in response to stress signals. Moreover, immature CD45 knockout (KO) cells showed defective motility, including reduced homing (both steady state and in response to stromal-derived factor 1) and reduced granulocyte colony-stimulating factor mobilization. These defects were associated with increased cell adhesion mediated by reduced matrix metalloproteinase 9 secretion and imbalanced Src kinase activity. Poor mobilization of CD45KO progenitors by the receptor activator of nuclear factor kappaB ligand, and impaired modulation of the endosteal components osteopontin and stem cell factor, suggested defective osteoclast function. Indeed, CD45KO osteoclasts exhibited impaired bone remodeling and abnormal morphology, which we attributed to defective cell fusion and Src function. This led to irregular distribution of metaphyseal bone trabecules, a region enriched with stem cell niches. Consequently, CD45KO mice had less primitive cells in the BM and increased numbers of these cells in the spleen, yet with reduced homing and repopulation potential. Uncoupling environmental and intrinsic defects in chimeric mice, we demonstrated that CD45 regulates progenitor movement and retention by influencing both the hematopoietic and nonhematopoietic compartments.

A crosstalk between intracellular CXCR7 and CXCR4 involved in rapid CXCL12-triggered integrin activation but not in chemokine-triggered motility of human T lymphocytes and CD34+ cells

The chemokine CXCL12 promotes migration of human leukocytes, hematopoietic progenitors, and tumor cells. The binding of CXCL12 to its receptor CXCR4 triggers Gi protein signals for motility and integrin activation in many cell types. CXCR7 is a second, recently identified receptor for CXCL12, but its role as an intrinsic G-protein-coupled receptor (GPCR) has been debated. We report that CXCR7 fails to support on its own any CXCL12-triggered integrin activation or motility in human T lymphocytes or CD34(+) progenitors. CXCR7 is also scarcely expressed on the surface of both cell types and concentrates right underneath the plasma membrane with partial colocalization in early endosomes. Nevertheless, various specific CXCR7 blockers get access to this pool and attenuate the ability of CXCR4 to properly rearrange by surface-bound CXCL12, a critical step in the ability of the GPCR to trigger optimal CXCL12-mediated stimulation of integrin activation in T lymphocytes as well as in CD34(+) cells. In contrast, CXCL12-triggered CXCR4 signaling to early targets, such as Akt as well as CXCR4-mediated chemotaxis, is insensitive to identical CXCR7 blocking. Our findings suggest that although CXCR7 is not an intrinsic signaling receptor for CXCL12 on lymphocytes or CD34(+) cells, its blocking can be useful for therapeutic interference with CXCR4-mediated activation of integrins.

Catecholaminergic neurotransmitters regulate migration and repopulation of immature human CD34+ cells through Wnt signaling

Catecholamines are important regulators of homeostasis, yet their functions in hematopoiesis are poorly understood. Here we report that immature human CD34+ cells dynamically expressed dopamine and beta2-adrenergic receptors, with higher expression in the primitive CD34+CD38(lo) population. The myeloid cytokines G-CSF and GM-CSF upregulated neuronal receptor expression on immature CD34+ cells. Treatment with neurotransmitters increased the motility, proliferation and colony formation of human progenitor cells, correlating with increased polarity, expression of the metalloproteinase MT1-MMP and activity of the metalloproteinase MMP-2. Treatment with catecholamines enhanced human CD34+ cell engraftment of NOD-SCID mice through Wnt signaling activation and increased cell mobilization and bone marrow Sca-1+c-Kit+Lin- cell numbers. Our results identify new functions for neurotransmitters and myeloid cytokines in the direct regulation of human and mouse progenitor cell migration and development.

The multiple roles of osteoclasts in host defense: bone remodeling and hematopoietic stem cell mobilization

Bone remodeling by bone-forming osteoblasts and bone-resorbing osteoclasts dynamically alters the bone inner wall and the endosteum region, which harbors osteoblastic niches for hematopoietic stem cells. Investigators have recently elucidated mechanisms of recruitment and mobilization; these mechanisms consist of stress signals that drive migration of leukocytes and progenitor cells from the bone marrow reservoir to the circulation and drive their homing to injured tissues as part of host defense and repair. The physical bone marrow vasculature barrier that is crossed by mobilized cells actively transmits chemotactic signals between the blood and the bone marrow, facilitating organ communication and cell trafficking. Osteoclasts play a dual role in regulation of bone resorption and homeostatic release or stress-induced mobilization of hematopoietic stem/progenitor cells. In this review, we discuss the orchestrated interplay between bone remodeling, the immune system, and the endosteal stem cell niches in the context of stem cell proliferation and migration during homeostasis, which are accelerated during alarm situations.

p53 Attenuates cancer cell migration and invasion through repression of SDF-1/CXCL12 expression in stromal fibroblasts

The p53 tumor suppressor acts as a major barrier against cancer. To a large extent, this is due to its ability to maintain genome stability and to eliminate cancer cells from the replicative pool through cell-autonomous mechanisms. However, in addition to its well-documented functions within the malignant cancer cell, p53 can also exert non-cell-autonomous effects that contribute to tumor suppression. We now report that p53 can suppress the production of the chemokine SDF-1 in cultured fibroblasts of both human and mouse origin. This is due to a p53-mediated down-regulation of SDF-1 mRNA, which can be exacerbated on activation of p53 by the drug Nutlin-3. SDF-1 promotes the migration and invasiveness of cells that express its cognate receptor CXCR4. Indeed, medium conditioned by p53-deficient fibroblasts induces cancer cells towards increased directional migration and invasiveness, which are largely reversed by CXCR4 antagonist peptides. Because SDF-1 produced by stromal fibroblasts plays an important role in cancer progression and metastasis, our findings suggest that the ability of p53 to suppress stromal SDF-1 production may be an important mechanism whereby it does its non-cell-autonomous tumor suppressor function.

Functional CXCR4-Expressing Microparticles and SDF-1 Correlate with Circulating Acute Myelogenous Leukemia Cells

Stromal cell-derived factor-1 (SDF-1/CXCL12) and its receptor CXCR4 are implicated in the pathogenesis and prognosis of acute myelogenous leukemia (AML). Cellular microparticles, submicron vesicles shed from the plasma membrane of various cells, are also associated with human pathology. In the present study, we investigated the putative relationships between the SDF-1/CXCR4 axis and microparticles in AML. We detected CXCR4-expressing microparticles (CXCR4(+) microparticles) in the peripheral blood and bone marrow plasma samples of normal donors and newly diagnosed adult AML patients. In samples from AML patients, levels of CXCR4(+) microparticles and total SDF-1 were elevated compared with normal individuals. The majority of CXCR4(+) microparticles in AML patients were CD45(+), whereas in normal individuals, they were mostly CD41(+). Importantly, we found a strong correlation between the levels of CXCR4(+) microparticle and WBC count in the peripheral blood and bone marrow plasma obtained from the AML patients. Of interest, levels of functional, noncleaved SDF-1 were reduced in these patients compared with normal individuals and also strongly correlated with the WBC count. Furthermore, our data indicate NH(2)-terminal truncation of the CXCR4 molecule in the microparticles of AML patients. However, such microparticles were capable of transferring the CXCR4 molecule to AML-derived HL-60 cells, enhancing their migration to SDF-1 in vitro and increasing their homing to the bone marrow of irradiated NOD/SCID/beta2m(null) mice. The CXCR4 antagonist AMD3100 reduced these effects. Our findings suggest that functional CXCR4(+) microparticles and SDF-1 are involved in the progression of AML. We propose that their levels are potentially valuable as an additional diagnostic AML variable.

Gene expression profiles of AML derived stem cells; similarity to hematopoietic stem cells

Tumors contain a fraction of cancer stem cells that maintain the propagation of the disease. The CD34(+)CD38(-) cells, isolated from acute myeloid leukemia (AML), were shown to be enriched leukemic stem cells (LSC). We isolated the CD34(+)CD38(-) cell fraction from AML and compared their gene expression profiles to the CD34(+)CD38(+) cell fraction, using microarrays. We found 409 genes that were at least twofold over- or underexpressed between the two cell populations. These include underexpression of DNA repair, signal transduction and cell cycle genes, consistent with the relative quiescence of stem cells, and chromosomal aberrations and mutations of leukemic cells. Comparison of the LSC expression data to that of normal hematopoietic stem cells (HSC) revealed that 34% of the modulated genes are shared by both LSC and HSC, supporting the suggestion that the LSC originated within the HSC progenitors. We focused on the Notch pathway since Jagged-2, a Notch ligand was found to be overexpressed in the LSC samples. We show that DAPT, an inhibitor of gamma-secretase, a protease that is involved in Jagged and Notch signaling, inhibits LSC growth in colony formation assays. Identification of additional genes that regulate LSC self-renewal may provide new targets for therapy.