Phenotypic and Functional Changes Induced at the Clonal Level in Hematopoietic Stem Cells After 5-Fluorouracil Treatment

Deregulation of the CXCL12/CXCR4 axis in methotrexate chemotherapy-induced damage and recovery of the bone marrow microenvironment

Cancer chemotherapy disrupts the bone marrow (BM) microenvironment affecting steady-state proliferation, differentiation and maintenance of haematopoietic (HSC) and stromal stem and progenitor cells; yet the underlying mechanisms and recovery potential of chemotherapy-induced myelosuppression and bone loss remain unclear. While the CXCL12/CXCR4 chemotactic axis has been demonstrated to be critical in maintaining interactions between cells of the two lineages and progenitor cell homing to regions of need upon injury, whether it is involved in chemotherapy-induced BM damage and repair is not clear. Here, a rat model of chemotherapy treatment with the commonly used antimetabolite methotrexate (MTX) (five once-daily injections at 0.75 mg/kg/day) was used to investigate potential roles of CXCL12/CXCR4 axis in damage and recovery of the BM cell pool. Methotrexate treatment reduced marrow cellularity, which was accompanied by altered CXCL12 protein levels (increased in blood plasma but decreased in BM) and reduced CXCR4 mRNA expression in BM HSC cells. Accompanying the lower marrow CXCL12 protein levels (despite its increased mRNA expression in stromal cells) was increased gene and protein levels of metalloproteinase MMP-9 in bone and BM. Furthermore, recombinant MMP-9 was able to degrade CXCL12 in vitro. These findings suggest that MTX chemotherapy transiently alters BM cellularity and composition and that the reduced cellularity may be associated with increased MMP-9 expression and deregulated CXCL12/CXCR4 chemotactic signalling.

Mobilization of hematopoietic stem/progenitor cells: general principles and molecular mechanisms

Hematopoietic stem/progenitor cell mobilization can be achieved by a variety of bone marrow niche modifications, although efficient mobilization requires simultaneous expansion of the stem/progenitor cell pool and niche modification. Many of the mechanisms involved in G-CSF-induced mobilization have been described. With regard to mobilization of hematopoietic stem/progenitor cells, challenges for the future include the analysis of genetic factors responsible for the great variability in mobilization responses, and the identification of predictors of mobilization efficiency, as well as the development of mobilizing schemes for poor mobilizers. Moreover, improved regimens for enhanced or even preferential mobilization of nonhematopoietic stem/progenitor cell types, and their therapeutic potential for endogenous tissue repair will be questions to be vigorously pursued in the near future.

Regulation of murine hematopoietic stem cell quiescence by Dmtf1

The cell-cycle status of hematopoietic stem cells (HSCs) is tightly regulated, most likely to balance maintenance of stem-cell status through quiescence and expansion/differentiation of the hematopoietic system. Tumor-suppressor genes (TSGs), with their cell cycle-regulatory functions, play important roles in HSC regulation. The cyclin-D binding myb-like transcription factor 1 (Dmtf1) was recently recognized as a TSG involved in human cancers by repressing oncogenic Ras/Raf signaling. However, the role of Dmtf1 in the hematopoietic system is entirely unknown. In the present study, we demonstrate that Dmtf1 regulates HSC function under both steady-state and stress conditions. Dmtf1(-/-) mice showed increased blood cell counts in multiple parameters, and their progenitor cells had increased proliferation and accelerated cell-cycle progression. In addition, long-term HSCs from Dmtf1(-/-) mice had a higher self-renewal capacity that was clearly demonstrated in secondary recipients in serial transplantation studies. Dmtf1(-/-) BM cells showed hyper proliferation after 5-fluorouracil-induced myeloablation. Steady-state expression and Induction of CDKN1a (p21) and Arf were impaired in HSCs from Dmtf1(-/-) mice. The function of Dmtf1 was mediated by both Arf-dependent and Arf-independent pathways. Our results implicate Dmtf1 in the regulation of HSC function through novel cell cycle-regulatory mechanisms.

Essential roles for Pot1b in HSC self-renewal and survival

Maintenance of mammalian telomeres requires both the enzyme telomerase and shelterin, which protect telomeres from inappropriately activating DNA damage response checkpoints. Dyskeratosis congenita is an inherited BM failure syndrome disorder because of defects in telomere maintenance. We have previously shown that deletion of the shelterin component Pot1b in the setting of telomerase haploinsufficiency results in rapid telomere shortening and fatal BM failure in mice, eliciting phenotypes that strongly resemble human syskeratosis congenita. However, it was unclear why BM failure occurred in the setting of Pot1b deletion. In this study, we show that Pot1b plays an essential role in HSC survival. Deletion of Pot1b results in increased apoptosis, leading to severe depletion of the HSC reserve. BM from Pot1b(Δ/Δ) mice cannot compete with BM from wild-type mice to provide multilineage reconstitution, indicating that there is an intrinsic requirement for Pot1b the maintenance of HSC function in vivo. Elimination of the p53-dependent apoptotic function increased HSC survival and significantly extended the lifespan of Pot1b-null mice deficient in telomerase function. Our results document for the first time the essential role of a component of the shelterin complex in the maintenance of HSC and progenitor cell survival.

Ontogeny stage-independent and high-level clonal expansion in vitro of mouse hematopoietic stem cells stimulated by an engineered NUP98-HOX fusion transcription factor

Achieving high-level expansion of hematopoietic stem cells (HSCs) in vitro will have an important clinical impact in addition to enabling elucidation of their regulation. Here, we couple the ability of engineered NUP98-HOXA10hd expression to stimulate > 1000-fold net expansions of murine HSCs in 10-day cultures initiated with bulk lin(-)Sca-1(+)c-kit(+) cells, with strategies to purify fetal and adult HSCs and analyze their expansion clonally. We find that NUP98-HOXA10hd stimulates comparable expansions of HSCs from both sources at ∼ 60% to 90% unit efficiency in cultures initiated with single cells. Clonally expanded HSCs consistently show balanced long-term contributions to the lymphoid and myeloid lineages without evidence of leukemogenic activity. Although effects on fetal and adult HSCs were indistinguishable, NUP98-HOXA10hd-transduced adult HSCs did not thereby gain a competitive advantage in vivo over freshly isolated fetal HSCs. Live-cell image tracking of single transduced HSCs cultured in a microfluidic device indicates that NUP98-HOXA10hd does not affect their proliferation kinetics, and flow cytometry confirmed the phenotype of normal proliferating HSCs and allowed reisolation of large numbers of expanded HSCs at a purity of 25%. These findings point to the effects of NUP98-HOXA10hd on HSCs in vitro being mediated by promoting self-renewal and set the stage for further dissection of this process.

PR-domain-containing Mds1-Evi1 is critical for long-term hematopoietic stem cell function

The Mds1 and Evi1 complex locus (Mecom) gives rise to several alternative transcripts implicated in leukemogenesis. However, the contribution that Mecom-derived gene products make to normal hematopoiesis remains largely unexplored. To investigate the role of the upstream transcription start site of Mecom in adult hematopoiesis, we created a mouse model with a lacZ knock-in at this site, termed ME(m1), which eliminates Mds1-Evi1 (ME), the longer, PR-domain-containing isoform produced by the gene (also known as PRDM3). β-galactosidase-marking studies revealed that, within hematopoietic cells, ME is exclusively expressed in the stem cell compartment. ME deficiency leads to a reduction in the number of HSCs and a complete loss of long-term repopulation capacity, whereas the stem cell compartment is shifted from quiescence to active cycling. Genetic exploration of the relative roles of endogenous ME and EVI1 isoforms revealed that ME preferentially rescues long-term HSC defects. RNA-seq analysis in Lin(-)Sca-1(+)c-Kit(+) cells (LSKs) of ME(m1) documents near complete silencing of Cdkn1c, encoding negative cell-cycle regulator p57-Kip2. Reintroduction of ME into ME(m1) LSKs leads to normalization of both p57-Kip2 expression and growth control. Our results clearly demonstrate a critical role of PR-domain-containing ME in linking p57-kip2 regulation to long-term HSC function.

Dioscorea phytocompounds enhance murine splenocyte proliferation ex vivo and improve regeneration of bone marrow cells in vivo

Specific cytokines have been tested clinically for immunotherapy of cancers; however, cytotoxicity has often impaired their usefulness. Consequently, alternative approaches are increasingly desirable. Dioscorea spp. tuber is a widely used traditional Chinese medicinal herb claimed to confer immunostimulatory activity. In this study, we evaluated Dioscorea as an adjuvant therapy for use alongside chemotherapy for cancer. Phytocompounds from Dioscorea tubers were ethanol fractioned and used for ex vivo splenocyte proliferation assay or in vivo force-feeding of mice pre-treated with the chemotherapy agent 5-fluorouracil. Co-treatment with a 50–75% ethanol-partitioned fraction of the tuber extract of D. batatas (DsCE-II) and interleukin (IL)-2 resulted in a significantly higher rate of murine splenocyte cell proliferation ex vivo than treatment with DsCE-II or IL-2 alone. This DsCE-II fraction, which contains a polysaccharide with a high proportion of β-1,4-linkage mannose (≥64%), also promoted the regeneration of specific progenitor cell populations in damaged bone marrow tissues of 5-fluorouracil-treated mice. Colony-forming unit (CFU) analyses demonstrated that the population of CFU-GM cells, but not CFU-GEMM or BFU-E cells, preferentially recovered to ~67% in the bone marrow of immune-suppressed mice fed with DsCE-II. DsCE-II efficacy level was ~85% of that obtained by subcutaneous administration of recombinant G-CSF proteins (5 μg kg⁻¹) in mice tested in parallel. This study suggests that the DsCE-II fraction of D. batatas extract may be considered for further development as a dietary supplement for use alongside chemotherapy during cancer treatment.

Using chemotherapeutic drugs to trigger hematopoietic stem cell activation and determine hematological toxicity

Hematopoietic stem cells (HSCs) replenish blood cells throughout the lifetime of an animal. Maintaining the HSC pool is of paramount importance to the survival of the organism. Thus, when treating a patient or an animal with chemotherapeutic agents, all care must be taken to ensure that the HSC pool is minimally damaged while simultaneously eradicating the target population. Discussed here is a method to assess the potency of chemotherapeutic drugs on the hematopoietic system, their ability to activate the HSC compartment, and the damage done to the HSC pool.

Quiescence regulators for hematopoietic stem cell

Hematopoietic stem cell (HSC) either stays in quiescence or proliferates toward differentiation for the production of mature blood cells, or toward self-renewal for giving rise to itself. In order to both maintain a supply of mature blood cells and not exhaust HSCs throughout the lifetime of an individual, under steady state, most HSCs remain quiescent and only a small number enter the cell cycle. Quiescence of HSCs is not only critical for protecting the stem cell compartment and sustaining stem cell pools over long periods, but it is also critical for protecting stem cells by minimizing their accumulation of replication-associated mutations. The balance between quiescence and proliferation is tightly controlled by both HSC-intrinsic and -extrinsic mechanisms. In recent years, through reductionistic strategies, a wide variety of molecules or pathways critical for HSC quiescence regulation have been identified. This regulation network involves both positive and negative regulators. Understanding quiescence regulation in HSC is of great importance not only for understanding the physiological foundation of HSCs, but also for understanding the pathophysiological origins of many related disorders. In this article, I will briefly review the current advance in the quiescence regulators for the HSCs.

Inflammatory signals regulate hematopoietic stem cells

Hematopoietic stem cells (HSCs) are the progenitors of all blood and immune cells, yet their role in immunity is not well understood. Most studies have focused on the ability of committed lymphoid and myeloid precursors to replenish immune cells during infection. Recent studies, however, have indicated that HSCs also proliferate in response to systemic infection and replenish effector immune cells. Inflammatory signaling molecules including interferons, tumor necrosis factor-α and Toll-like receptors are essential to the HSC response. Observing the biology of HSCs through the lens of infection and inflammation has led to the discovery of an array of immune-mediators that serve crucial roles in HSC regulation and function.

p53-mediated hematopoietic stem and progenitor cell competition

Cell competition was originally described in Drosophila as a process for selection of the fittest cells. It is likely to play an important role in tissue homeostasis in all metazoans, but little is known about its role and regulation in mammals. By using genetic mosaic mouse models and bone marrow chimeras, we describe here a form of cell competition that selects for the least damaged cells. This competition is controlled by p53 but is distinct from the classical p53-mediated DNA damage response: it persists for months, is specific to the hematopoietic stem and progenitor cells, and depends on the relative rather than absolute level of p53 in competing cells. The competition appears to be mediated by a non-cell-autonomous induction of growth arrest and senescence-related gene expression in outcompeted cells with higher p53 activity. p53-mediated cell competition of this type could potentially contribute to the clonal expansion of incipient cancer cells.

Awaking stem cells from dormancy: growing old and fighting cancer

It is generally accepted that a distinguishing property of stem cells (SCs), as compared to their more differentiated progenitors, is that of infrequent division, often referred to as ‘quiescence’. As regards hematopoietic stem cells (HSC), their resistance to antiproliferative drugs supports this notion. Maintenance of quiescence is thought to be critical for the preservation of HSCs' function.

Slug deficiency enhances self-renewal of hematopoietic stem cells during hematopoietic regeneration

Both extrinsic and intrinsic mechanisms tightly govern hematopoietic stem cell (HSC) decisions of self-renewal and differentiation. However, transcription factors that can selectively regulate HSC self-renewal division after stress remain to be identified. Slug is an evolutionarily conserved zinc-finger transcription factor that is highly expressed in primitive hematopoietic cells and is critical for the radioprotection of these key cells. We studied the effect of Slug in the regulation of HSCs in Slug-deficient mice under normal and stress conditions using serial functional assays. Here, we show that Slug deficiency does not disturb hematopoiesis or alter HSC homeostasis and differentiation in bone marrow but increases the numbers of primitive hematopoietic cells in the extramedullary spleen site. Deletion of Slug enhances HSC repopulating potential but not its homing and differentiation ability. Furthermore, Slug deficiency increases HSC proliferation and repopulating potential in vivo after myelosuppression and accelerates HSC expansion during in vitro culture. Therefore, we propose that Slug is essential for controlling the transition of HSCs from relative quiescence under steady-state condition to rapid proliferation under stress conditions. Our data suggest that inhibition of Slug in HSCs may present a novel strategy for accelerating hematopoietic recovery, thus providing therapeutic benefits for patients after clinical myelosuppressive treatment.

Necdin restricts proliferation of hematopoietic stem cells during hematopoietic regeneration

Hematopoietic stem cell (HSC) proliferation is tightly regulated by a poorly understood complex of positive and negative cell-cycle regulatory mechanisms. Necdin (Ndn) is an evolutionally conserved multifunctional protein that has been implicated in cell-cycle regulation of neuronal cells. Here, we provide evidence that necdin plays an important role in restricting excessive HSC proliferation during hematopoietic regeneration. We identify Ndn as being preferentially expressed in the HSC population on the basis of gene expression profiling and demonstrate that mice deficient in Ndn show accelerated recovery of the hematopoietic system after myelosuppressive injury, whereas no overt abnormality is seen in steady-state hematopoiesis. In parallel, after myelosuppression, Ndn-deficient mice exhibit an enhanced number of proliferating HSCs. Based on these findings, we propose that necdin functions in a negative feedback loop that prevents excessive proliferation of HSCs during hematopoietic regeneration. These data suggest that the inhibition of necdin after clinical myelosuppressive treatment (eg, chemotherapy, HSC transplantation) may provide therapeutic benefits by accelerating hematologic recovery.

Genetic models to study quiescent stem cells and their niches

Hematopoietic stem cells (HSC) have been defined by their ability to establish long-term hematopoiesis in myelo-ablated hosts. Prospective isolation using combinations of cell-surface markers and/or dye exclusion can yield highly purified and nearly homogeneous phenotypically defined cells that repopulate irradiated hosts. Although highly informative, these types of analyses may not necessarily reflect ongoing homeostatic hematopoiesis. HSCs are also described as being quiescent. This has been demonstrated by cell cycle analysis of phenotypically defined HSCs. Some studies have challenged the existence of truly quiescent HSCs, suggesting that they continuously cycle, albeit with very slow kinetics. Here we present a pulse-chase system based on the controllable incorporation of H2B-GFP into nucleosomes, which allows the identification, purification, and functional analysis of viable label-retaining cells. Our data complement and extend recent studies using similar strategies. These, together with our present studies, find a rare, quiescent or dormant subset within the population of stringently defined HSC phenotypes. To date, three types of niches, endosteal, vascular, and reticular, have been described; herein we review the cellular and spatial nature of these microenvironments. We propose that HSC label-retention combined with genetically manipulated stem cell niches will allow us to determine their anatomical architecture, to address HSC cell fate proliferation kinetics, and to begin to dissect the molecular cross talk among stem cells and niche cells in vivo during both normal and perturbed homeostasis.

Balancing dormant and self-renewing hematopoietic stem cells

The mouse hematopoietic stem cell (HSC) is probably the best-understood somatic stem cell in higher organisms. Recent studies have shown that the highest self-renewal potential is most likely contained within an exceedingly small number of deeply dormant bone marrow HSCs. These stem cells are housed in individual niches that preserve their dormancy via signaling molecules such as Thrombopoietin, Angiopoietins, and Stem Cell Factor. In response to injury cues, dormant HSCs are efficiently activated and produce numerous progenitors and mature cells. A series of intracellular regulatory molecules including FoxOs, mTORC1, Fbw7, Egr1, Pbx1, pRb, c-Cbl, Myc, and Bmi1 mediate the processes of dormancy, cycling, self-renewal, differentiation, and survival, all of which control the behavior of HSCs.

Loss of the Rho GTPase activating protein p190-B enhances hematopoietic stem cell engraftment potential

Hematopoietic stem cell (HSC) engraftment is a multistep process involving HSC homing to bone marrow, self-renewal, proliferation, and differentiation to mature blood cells. Here, we show that loss of p190-B RhoGTPase activating protein, a negative regulator of Rho GTPases, results in enhanced long-term engraftment during serial transplantation. This effect is associated with maintenance of functional HSC-enriched cells. Furthermore, loss of p190-B led to marked improvement of HSC in vivo repopulation capacity during ex vivo culture without altering proliferation and multilineage differentiation of HSC and progeny. Transcriptional analysis revealed that p190-B deficiency represses the up-regulation of p16(Ink4a) in HSCs in primary and secondary transplantation recipients, providing a possible mechanism of p190-B-mediated HSC functions. Our study defines p190-B as a critical transducer element of HSC self-renewal activity and long-term engraftment, thus suggesting that p190-B is a target for HSC-based therapies requiring maintenance of engraftment phenotype.

IFNalpha activates dormant haematopoietic stem cells in vivo

Maintenance of the blood system is dependent on dormant haematopoietic stem cells (HSCs) with long-term self-renewal capacity. After injury these cells are induced to proliferate to quickly re-establish homeostasis. The signalling molecules promoting the exit of HSCs out of the dormant stage remain largely unknown. Here we show that in response to treatment of mice with interferon-alpha (IFNalpha), HSCs efficiently exit G(0) and enter an active cell cycle. HSCs respond to IFNalpha treatment by the increased phosphorylation of STAT1 and PKB/Akt (also known as AKT1), the expression of IFNalpha target genes, and the upregulation of stem cell antigen-1 (Sca-1, also known as LY6A). HSCs lacking the IFNalpha/beta receptor (IFNAR), STAT1 (ref. 3) or Sca-1 (ref. 4) are insensitive to IFNalpha stimulation, demonstrating that STAT1 and Sca-1 mediate IFNalpha-induced HSC proliferation. Although dormant HSCs are resistant to the anti-proliferative chemotherapeutic agent 5-fluoro-uracil, HSCs pre-treated (primed) with IFNalpha and thus induced to proliferate are efficiently eliminated by 5-fluoro-uracil exposure in vivo. Conversely, HSCs chronically activated by IFNalpha are functionally compromised and are rapidly out-competed by non-activatable Ifnar(-/-) cells in competitive repopulation assays. Whereas chronic activation of the IFNalpha pathway in HSCs impairs their function, acute IFNalpha treatment promotes the proliferation of dormant HSCs in vivo. These data may help to clarify the so far unexplained clinical effects of IFNalpha on leukaemic cells, and raise the possibility for new applications of type I interferons to target cancer stem cells.

Membrane-anchored uPAR regulates the proliferation, marrow pool size, engraftment, and mobilization of mouse hematopoietic stem/progenitor cells

The mechanisms of BM hematopoietic stem/progenitor cell (HSPC) adhesion, engraftment, and mobilization remain incompletely identified. Here, using WT and transgenic mice, we have shown that membrane-anchored plasminogen activator, urokinase receptor (MuPAR) marks a subset of HSPCs and promotes the preservation of the size of this pool of cells in the BM. Loss or inhibition of MuPAR increased HSPC proliferation and impaired their homing, engraftment, and adhesion to the BM microenvironment. During mobilization, MuPAR was inactivated by plasmin via proteolytic cleavage. Cell-autonomous loss of the gene encoding MuPAR also impaired long-term engraftment and multilineage repopulation in primary and secondary recipient mice. These findings identify MuPAR and plasmin as regulators of the proliferation, marrow pool size, homing, engraftment, and mobilization of HSPCs and possibly also of HSCs.

The p47 GTPase Lrg-47 (Irgm1) links host defense and hematopoietic stem cell proliferation

Hematopoietic stem cells (HSCs) are self-renewing bone marrow cells that give rise to all blood lineages and retain a remarkable capacity to proliferate in response to insult. Although some controls on HSC activation are known, little is understood about how this process is linked to natural signals. We report that the interferon-inducible GTPase Lrg-47 (Irgm1), previously shown to play a critical role in host defense, inhibits baseline HSC proliferation and is required for a normal HSC response to chemical and infectious stimuli. Overproliferating Lrg-47(-/-) HSCs are severely impaired in functional repopulation assays, and when challenged with hematopoietic ablation by 5-fluorouracil or infection with Mycobacterium avium, Lrg-47(-/-) mice fail to achieve the expected expansion response in stem and progenitor cell populations. Our results establish a link between the response to infection and HSC activation and demonstrate a novel function for a member of the p47 GTPase family.

Both alleles of PSF1 are required for maintenance of pool size of immature hematopoietic cells and acute bone marrow regeneration

Hematopoietic stem cells (HSCs) have a very low rate of cell division in the steady state; however, under conditions of hematopoietic stress, these cells can begin to proliferate at high rates, differentiate into mature hematopoietic cells, and rapidly reconstitute ablated bone marrow (BM). Previously, we isolated a novel evolutionarily conserved DNA replication factor, PSF1 (partner of SLD5-1), from an HSC-specific cDNA library. In the steady state, PSF1 is expressed predominantly in CD34+KSL (c-kit+/Sca-1+/Lineage−) cells and progenitors, whereas high levels of PSF1 expression are induced in KSL cells after BM ablation. In 1-year-old PSF1+/− mice, the pool size of stem cells and progenitors is decreased. Whereas young PSF1+/− mutant mice develop normally, are fertile, and have no obvious differences in hematopoiesis in the steady state compared with wild-type mice, intravenous injection of 5-fluorouracil (5-FU) is lethal in PSF1+/− mice, resulting from a delay in induction of HSC proliferation during ablated BM reconstitution. Overexpression studies revealed that PSF1 regulates molecular stability of other GINS components, including SLD5, PSF2, and PSF3. Our data indicate that PSF1 is required for acute proliferation of HSCs in the BM of mice.

Hematopoietic stem cell function and survival depend on c-Myc and N-Myc activity

Myc activity is emerging as a key element in acquisition and maintenance of stem cell properties. We have previously shown that c-Myc deficiency results in accumulation of defective hematopoietic stem cells (HSCs) due to niche-dependent differentiation defects. Here we report that immature HSCs coexpress c-myc and N-myc mRNA at similar levels. Although conditional deletion of N-myc in the bone marrow does not affect hematopoiesis, combined deficiency of c-Myc and N-Myc (dKO) results in pancytopenia and rapid lethality. Interestingly, proliferation of HSCs depends on both myc genes during homeostasis, but is c-Myc/N-Myc independent during bone marrow repair after injury. Strikingly, while most dKO hematopoietic cells undergo apoptosis, only self-renewing HSCs accumulate the cytotoxic molecule Granzyme B, normally employed by the innate immune system, thereby revealing an unexpected mechanism of stem cell apoptosis. Collectively, Myc activity (c-Myc and N-Myc) controls crucial aspects of HSC function including proliferation, differentiation, and survival.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Gadd45 modulation of intrinsic and extrinsic stress responses in myeloid cells

Gadd45 proteins modulate signaling in response to physiological and environmental stressors. Expression of gadd45 genes is rapidly induced by different stressors, including differentiation-inducing cytokines and genotoxic stress. Induction of gadd45 genes at the onset of myeloid differentiation suggested that Gadd45 protein(s) play a role in hematopoiesis, yet no apparent abnormalities were observed in either the bone marrow (BM) or peripheral blood compartments of mice deficient for either gadd45a or gadd45b. However, under conditions of hematological stress, including acute stimulation with cytokines, myelo-ablation and inflammation, both gadd45a-deficient and gadd45b-deficient mice exhibited deficiencies. This is discussed within the context of what is known about Gadd45 proteins in stress signaling, hematopoietic development and the innate immune response. Furthermore, myeloid enriched BM cells from gadd45a and gadd45b deficient mice were observed to be more sensitive to ultraviolet radiation (UVC), VP-16 and daunorubicin (DNR) induced apoptosis compared to wild-type (WT) cells, displaying defective G2/M arrest following exposure to UVC and VP-16, but not to DNR. Novel mechanisms that mediate the pro-survival functions of Gadd45 in hematopoietic cells following UV irradiation were demonstrated, involving activation of the Gadd45a-p38-NF-kappaB survival pathway and Gadd45b mediated inhibition of the stress response MKK4-JNK apoptotic pathway. The ramifications regarding the pathogenesis of different leukemias and the response of normal and malignant hematopoietic cells to chemo- and radiation-therapy, as well as other challenges to the hematopoietic compartment, are discussed.

Hematopoietic stem cells reversibly switch from dormancy to self-renewal during homeostasis and repair

Bone marrow hematopoietic stem cells (HSCs) are crucial to maintain lifelong production of all blood cells. Although HSCs divide infrequently, it is thought that the entire HSC pool turns over every few weeks, suggesting that HSCs regularly enter and exit cell cycle. Here, we combine flow cytometry with label-retaining assays (BrdU and histone H2B-GFP) to identify a population of dormant mouse HSCs (d-HSCs) within the lin(-)Sca1+cKit+CD150+CD48(-)CD34(-) population. Computational modeling suggests that d-HSCs divide about every 145 days, or five times per lifetime. d-HSCs harbor the vast majority of multilineage long-term self-renewal activity. While they form a silent reservoir of the most potent HSCs during homeostasis, they are efficiently activated to self-renew in response to bone marrow injury or G-CSF stimulation. After re-establishment of homeostasis, activated HSCs return to dormancy, suggesting that HSCs are not stochastically entering the cell cycle but reversibly switch from dormancy to self-renewal under conditions of hematopoietic stress.

The endothelial antigen ESAM marks primitive hematopoietic progenitors throughout life in mice

Although recent advances have enabled hematopoietic stem cells (HSCs) to be enriched to near purity, more information about their characteristics will improve our understanding of their development and stage-related functions. Here, using microarray technology, we identified endothelial cell-selective adhesion molecule (ESAM) as a novel marker for murine HSCs in fetal liver. Esam was expressed at high levels within a Rag1(-) c-kit(Hi) Sca1(+) HSC-enriched fraction, but sharply down-regulated with activation of the Rag1 locus, a valid marker for the most primitive lymphoid progenitors in E14.5 liver. The HSC-enriched fraction could be subdivided into 2 on the basis of ESAM levels. Among endothelial antigens on hematopoietic progenitors, ESAM expression showed intimate correlation with HSC activity. The ESAM(Hi) population was highly enriched for multipotent myeloid-erythroid progenitors and primitive progenitors with lymphopoietic activity, and exclusively reconstituted long-term lymphohematopoiesis in lethally irradiated recipients. Tie2(+) c-kit(+) lymphohematopoietic cells in the E9.5-10.5 aorta-gonad-mesonephros region also expressed high levels of ESAM. Furthermore, ESAM was detected on primitive hematopoietic progenitors in adult bone marrow. Interestingly, ESAM expression in the HSC-enriched fraction was up-regulated in aged mice. We conclude that ESAM marks HSC in murine fetal liver and will facilitate studies of hematopoiesis throughout life.

ADAR1 is essential for the maintenance of hematopoiesis and suppression of interferon signaling

The deaminase ADAR1 edits adenosines in nuclear transcripts of nervous tissue and is required in the fetal liver of the developing mouse embryo. Here we show by inducible gene disruption in mice that ADAR1 is essential for maintenance of both fetal and adult hematopoietic stem cells. Loss of ADAR1 in hematopoietic stem cells led to global upregulation of type I and II interferon-inducible transcripts and rapid apoptosis. Our findings identify ADAR1 as an essential regulator of hematopoietic stem cell maintenance and suppressor of interferon signaling that may protect organisms from the deleterious effects of interferon activation associated with many pathological processes, including chronic inflammation, autoimmune disorders and cancer.

Treatment of mice with the Ah receptor agonist and human carcinogen dioxin results in altered numbers and function of hematopoietic stem cells

The aryl hydrocarbon receptor (AhR) mediates the carcinogenicity of a family of environmental contaminants, the most potent being 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Increased incidence of lymphoma and leukemia in humans is associated with TCDD exposure. Although AhR activation by TCDD has profound effects on the immune system, precise cellular and molecular mechanisms have yet to be determined. These studies tested the hypothesis that alteration of marrow populations following treatment of mice with TCDD is due to an effect on hematopoietic stem cells (HSCs). Treatment with TCDD resulted in an increased number and proliferation of bone marrow (BM) populations enriched for HSCs. There was a time-dependent decrease in B-lineage cells with a concomitant increase in myeloid populations. The decrease in the B-cell lineage colony-forming unit-preB progenitors along with a transient increase in myeloid progenitors were consistent with a skewing of lineage development from lymphoid to myeloid populations. However, HSCs from TCDD-treated mice exhibited diminished capacity to reconstitute and home to marrow of irradiated recipients. AhR messenger RNA was expressed in progenitor subsets but is downregulated during HSC proliferation. This result was consistent with the lack of response following the exposure of 5-fluorouracil-treated mice to TCDD. The direct exposure of cultured BM cells to TCDD inhibited the growth of immature hematopoietic progenitor cells, but not more mature lineage-restricted progenitors. Overall, these data are consistent with the hypothesis that TCDD, through AhR activation, alters the ability of HSCs to respond appropriately to signals within the marrow microenvironment.

Long-term propagation of distinct hematopoietic differentiation programs in vivo

Heterogeneity in the differentiation behavior of hematopoietic stem cells is well documented but poorly understood. To investigate this question at a clonal level, we isolated a subpopulation of adult mouse bone marrow that is highly enriched for multilineage in vivo repopulating cells and transplanted these as single cells, or their short-term clonal progeny generated in vitro, into 352 recipients. Of the mice, 93 showed a donor-derived contribution to the circulating white blood cells for at least 4 months in one of four distinct patterns. Serial transplantation experiments indicated that two of the patterns were associated with extensive self-renewal of the original cell transplanted. However, within 4 days in vitro, the repopulation patterns subsequently obtained in vivo shifted in a clone-specific fashion to those with less myeloid contribution. Thus, primitive hematopoietic cells can maintain distinct repopulation properties upon serial transplantation in vivo, although these properties can also alter rapidly in vitro.

Wnt signaling in the niche enforces hematopoietic stem cell quiescence and is necessary to preserve self-renewal in vivo

Wingless (Wnt) is a potent morphogen demonstrated in multiple cell lineages to promote the expansion and maintenance of stem and progenitor cell populations. Wnt effects are highly context dependent, and varying effects of Wnt signaling on hematopoietic stem cells (HSCs) have been reported. We explored the impact of Wnt signaling in vivo, specifically in the context of the HSC niche by using an osteoblast-specific promoter driving expression of the paninhibitor of canonical Wnt signaling, Dickkopf1 (Dkk1). Here we report that Wnt signaling was markedly inhibited in HSCs and, unexpectedly given prior reports, reduction in HSC Wnt signaling resulted in reduced p21Cip1 expression, increased cell cycling, and a progressive decline in regenerative function after transplantation. This effect was microenvironment determined, but irreversible if the cells were transferred to a normal host. Wnt pathway activation in the niche is required to limit HSC proliferation and preserve the reconstituting function of endogenous hematopoietic stem cells.

c-myc in the hematopoietic lineage is crucial for its angiogenic function in the mouse embryo

The c-myc proto-oncogene, which is crucial for the progression of many human cancers, has been implicated in key cellular processes in diverse cell types, including endothelial cells that line the blood vessels and are critical for angiogenesis. The de novo differentiation of endothelial cells is known as vasculogenesis, whereas the growth of new blood vessels from pre-existing vessels is known as angiogenesis. To ascertain the function of c-myc in vascular development, we deleted c-myc in selected cell lineages. Embryos lacking c-myc in endothelial and hematopoietic lineages phenocopied those lacking c-myc in the entire embryo proper. At embryonic day (E) 10.5, both mutant embryos were grossly normal, had initiated primitive hematopoiesis, and both survived until E11.5-12.5, longer than the complete null. However, they progressively developed defective hematopoiesis and angiogenesis. The majority of embryos lacking c-myc specifically in hematopoietic cells phenocopied those lacking c-myc in endothelial and hematopoietic lineages, with impaired definitive hematopoiesis as well as angiogenic remodeling. c-myc is required for embryonic hematopoietic stem cell differentiation, through a cell-autonomous mechanism. Surprisingly, c-myc is not required for vasculogenesis in the embryo. c-myc deletion in endothelial cells does not abrogate endothelial proliferation, survival, migration or capillary formation. Embryos lacking c-myc in a majority of endothelial cells can survive beyond E12.5. Our findings reveal that hematopoiesis is a major function of c-myc in embryos and support the notion that c-myc functions in selected cell lineages rather than in a ubiquitous manner in mammalian development.

Steel factor coordinately regulates the molecular signature and biologic function of hematopoietic stem cells

Hematopoietic stem cells (HSCs) regenerated in vivo display sustained differences in their self-renewal and differentiation activities. Variations in Steel factor (SF) signaling are known to affect these functions in vitro, but the cellular and molecular mechanisms involved are not understood. To address these issues, we evaluated highly purified HSCs maintained in single-cell serum-free cultures containing 20 ng/mL IL-11 plus 1, 10, or 300 ng/mL SF. Under all conditions, more than 99% of the cells traversed a first cell cycle with similar kinetics. After 8 hours in the 10 or 300 ng/mL SF conditions, the frequency of HSCs remained unchanged. However, in the next 8 hours (ie, 6 hours before any cell divided), HSC integrity was sustained only in the 300 ng/mL SF cultures. The cells in these cultures also contained significantly higher levels of Bmi1, Lnk, and Ezh2 transcripts but not of several other regulators. Assessment of 21 first division progeny pairs further showed that only those generated in 300 ng/mL SF cultures contained HSCs and pairs of progeny with similar differentiation programs were not observed. Thus, SF signaling intensity can directly and coordinately alter the transcription factor profile and long-term repopulating ability of quiescent HSCs before their first division.

The effect of bleeding on hematopoietic stem cell cycling and self-renewal

Hematopoietic stem cells (HSCs) divide and give rise to more committed progenitors, which ultimately produce all lineages of blood cells. HSCs can be induced to enter the cell cycle in vitro and in vivo by stimulatory cytokines and in vivo by ablation of bone marrow (BM) cells with irradiation or chemotherapeutic agents. Although it has been postulated that rates of HSC proliferation increase with normal hematopoietic stresses, such as infection or hemorrhage, this hypothesis has never been directly tested. The ability to analyze HSCs prospectively by cell-surface phenotype c-kit(+), Thy1.1(lo), Sca-1(+), Linage(neg/lo) has allowed us to perform a detailed examination of the effects of bleeding on the cell cycle kinetics of HSCs. Our results demonstrate for the first time that HSCs in both the BM and the spleen proliferate and self-renew in response to tail-vein bleeding in mice. This response was suppressed when red blood cells, but not when white blood cells, were transferred after bleeding. Thus, regulators of HSC proliferation can sense and respond to red blood cell levels.

IL-12 facilitates both the recovery of endogenous hematopoiesis and the engraftment of stem cells after ionizing radiation

OBJECTIVE

Severe myelosuppression is a common side effect of radiotherapy or chemotherapy. Methods have been developed to protect patients by stimulating white blood cell or red blood cell recovery/production using growth factors such as G-CSF or EPO. However, there is no available means to stimulate the full-lineage blood cell recovery from severe myelosuppression. In this study, we used lethally or sublethally irradiated animal models to evaluate the hematopoiesis stimulating effect of IL-12.

MATERIALS AND METHODS

IL-12-treated lethally or sublethally irradiated animals were examined for the survival/lifespan, the function assays (bone marrow transplantation, CFU-S(12), CFC) of bone marrow cell subsets, and apoptosis assay.

RESULTS

Using a low dose of IL-12 (10 times lower than previously reported dose), 91.4% of lethally irradiated animals survived long term without adverse effects on the gastrointestinal (GI) system. The reconstituted hematopoietic system was derived from long-term reconstituting hematopoietic stem cells (LTR HSC), which reconstituted hematopoiesis both endogenously after lethal radiation and in secondary recipients by bone marrow transplantation (BMT). IL-12 significantly attenuated the decline of blood cell counts in sublethally irradiated animals. The IL-12-stimulated hematopoiesis recovery resulted in a full-lineage blood cell production, including white and red blood cells, and platelets. There was no detectable expression of IL-12 receptor on LTR HSC. In IL-12-treated animals, the number of Sca-1(+) cells was significantly higher than in animals without IL-12 treatment.

CONCLUSION

In this study, we showed a low dose of IL-12 has hematopoietic-protecting effects, which can attenuate severe myelosuppresion caused by lethal or sublethal irradiation. This study, together with previous studies showing IL-12 is also an anti-tumor and anti-angiogenic agent, suggest IL-12 may have clinical significance in cancer treatment and BMT.

The retinoblastoma tumor suppressor is a critical intrinsic regulator for hematopoietic stem and progenitor cells under stress

The retinoblastoma tumor suppressor protein (RB) plays important roles in the control of the cell division cycle. It is estimated that RB is dysfunctional/inactivated in up to 40% of human leukemias. The consequences of loss of RB on hematopoietic stem and progenitor cell (HSPC) function in vivo are incompletely understood. Here, we report that mice genetically deficient in Rb in all hematopoietic cells (Vav-Cre Rb knockout [KO] animals) showed altered contribution of distinct hematopoietic cell lineages to peripheral blood, bone marrow, and spleen; significantly increased extramedullary hematopoiesis in the spleen; and a 2-fold increase in the frequency of hematopoietic progenitor cells in peripheral blood. Upon competitive transplantation, HSPCs from Vav-Cre Rb KO mice contributed with an at least 4- to 6-fold less efficiency to hematopoiesis compared with control cells. HSPCs deficient in Rb presented with impaired cell-cycle exit upon stress-induced proliferation, which correlated with impaired function. In summary, Rb is critical for hematopoietic stem and progenitor cell function, localization, and differentiation.

Role of gadd45 in myeloid cells in response to hematopoietic stress

The gadd45 family of genes is rapidly induced by different stressors, including differentiation-inducing cytokines, and there is a large body of evidence that their cognate proteins are key players in cellular stress responses. Induction of gadd45 genes at the onset of myeloid differentiation suggested that Gadd45 protein(s) play a role in hematopoiesis, yet no apparent abnormalities were observed in either the bone marrow or peripheral blood compartments of mice deficient for either gadd45a or gadd45b. However, under conditions of hematological stress, including acute stimulation with cytokines, myelo-ablation and inflammation, both gadd45a-deficient and gadd45b-deficient mice exhibited deficiencies. This topic is discussed within the context of what is known about Gadd45 proteins in stress signaling, hematopoietic development and the innate immune response.

Molecular profiling of hematopoietic stem cells

Gene expression profiling using microarrays is a powerful method for studying the biology of hematopoietic stem cells (HSCs). Here, we present methods for activating HSCs with the chemotherapeutic drug 5-Fluorouracil, isolating HSCs from whole bone marrow, and performing microarray analysis. We also discuss quality control criteria for identifying good arrays and bioinformatics strategies for analyzing them. Using these methods, we have characterized the gene expression signatures of HSC quiescence and proliferation and have constructed a molecular model of HSC activation and self-renewal.

NF-E2-mediated enhancement of megakaryocytic differentiation and platelet production in vitro and in vivo

OBJECTIVE

NF-E2 is a prime regulator of megakaryocyte (MK) terminal differentiation and platelet release. By overexpressing the p45 subunit of NF-E2, we aim to increase the proportion of mature MKs and the potential for platelet production in vitro and in vivo.

METHODS

Retroviral vectors expressing p45-NF-E2 together with the enhanced green fluorescent protein (eGFP) were used to transduce murine bone marrow cells (BMCs). Aspects of MK differentiation, proliferation, proplatelet, and platelet production were evaluated.

RESULTS

Compared to controls, a higher proportion of BMCs overexpressing p45-NF-E2 were found to express the MK markers CD41, CD42a, and CD42b, with some effect on cell proliferation. Early MK differentiation, characterized by colony-forming unit (CFU)-MK formation, was enhanced by p45-NF-E2 overexpression at the expense of CFU-granulocyte macrophage development. An increased number of acetylcholinesterase(+) MKs was also observed in NF-E2(++) cultures. Although endomitosis was found not to be affected, the resultant upregulation of NF-E2 target genes was also followed by significant increases in proplatelet and functional platelet production. Transplantation of enriched MK progenitor cells overexpressing p45-NF-E2 into lethally irradiated mice resulted in a threefold increase in eGFP(+)/NF-E2(++) platelet production in vivo over 10 days, although no appreciable expansion in their number was observed over 32 days.

CONCLUSION

These results suggest that enforced expression of p45-NF-E2 selectively enhances many aspects of MK differentiation, including MK maturation, proplatelet formation, and platelet release. In addition, p45 overexpression increases MK commitment during early megakaryopoiesis, while inhibiting white blood cell differentiation.

Effects of bone marrow-derived cells on monocrotaline- and hypoxia-induced pulmonary hypertension in mice

BACKGROUND

Bone marrow -derived cells (BMDCs) can either limit or contribute to the process of pulmonary vascular remodeling. Whether the difference in their effects depends on the mechanism of pulmonary hypertension (PH) remains unknown.

OBJECTIVES

We investigated the effect of BMDCs on PH induced in mice by either monocrotaline or exposure to chronic hypoxia.

METHODS

Intravenous administration of the active monocrotaline metabolite (monocrotaline pyrrole, MCTp) to C57BL/6 mice induced PH within 15 days, due to remodeling of small distal vessels. Three days after the MCTp injection, the mice were injected with BMDCs harvested from femurs and tibias of donor mice treated with 5-fluorouracil (3.5 mg IP/animal) to deplete mature cells and to allow proliferation of progenitor cells.

RESULTS

BMDCs significantly attenuated PH as assessed by reductions in right ventricular systolic pressure (20 +/- 1 mmHg vs. 27 +/- 1 mmHg, P < or = 0.01), right ventricle weight/left ventricle+septum weight ratio (0.29 +/- 0.02 vs. 0.36 +/- 0.01, P < or = 0.03), and percentage of muscularized vessels (26.4% vs. 33.5%, P < or = 0.05), compared to control animals treated with irradiated BMDCs. Tracking cells from constitutive GFP-expressing male donor mice with anti-GFP antibodies or chromosome Y level measurement by quantitative real-time PCR showed BMDCs in the lung. In contrast, chronically hypoxic mice subjected to the same procedure failed to show improvement in PH.

CONCLUSION

These results show that BMDCs limit pulmonary vascular remodeling induced by vascular injury but not by hypoxia.

Cell cycle quiescence of early lymphoid progenitors in adult bone marrow

Lymphocyte production in bone marrow (BM) requires substantial cell division, but the relationship between largely quiescent stem cells and dividing lymphoid progenitors is poorly understood. Therefore, the proliferation and cell cycle status of murine hematopoietic progenitors that have just initiated the lymphoid differentiation program represented the focus of this study. Continuous bromo-2'-deoxyuridine (BrdU) incorporation and DNA/RNA analysis by flow cytometry revealed that a surprisingly large fraction of RAG-1(+)c-kit(hi) early lymphoid progenitors (ELPs) and RAG-1(+)c-kit(lo) pro-lymphocytes (Pro-Ls) in adult BM were in cell cycle quiescence. In contrast, their counterparts in 14-day fetal liver actively proliferated. Indeed, the growth fraction (cells in G(1)-S-G(2)-M phases) of fetal ELPs was on average 80% versus only 30% for adult ELPs. After 5-fluorouracil treatment, as many as 60% of the adult ELP-enriched population was in G(1)-S-G(2)-M and 34% incorporated BrdU in 6 hours. Transcripts for Bcl-2, p21Cip1/Waf1, and p27 Kip1 cell cycle regulatory genes correlated inversely well with proliferative activity. Interestingly, adult lymphoid progenitors in rebound had the high potential for B lymphopoiesis in culture typical of their fetal counterparts. Thus, lymphocyte production is sustained during adult life by quiescent primitive progenitors that divide intermittently. Some, but not all, aspects of the fetal differentiation program are reacquired after chemotherapy.

Hematopoietic cells from gadd45a-deficient and gadd45b-deficient mice exhibit impaired stress responses to acute stimulation with cytokines, myeloablation and inflammation

The gadd45 family of gene(s) is rapidly induced by genotoxic stress or by differentiation-inducing cytokines. Using bone marrow (BM) from gadd45a-/-, gadd45b-/- and wild-type (wt) mice, we investigated their role in stress responses of myeloid cells to acute stimulation with differentiating cytokines, myelotoxic agents and inflammatory substances. Bone marrow cells from gadd45a-/- and gadd45b-/- mice displayed compromised myeloid differentiation and higher apoptosis in vitro, following acute stimulation with a variety of differentiating cytokines. Intriguingly, gadd45a-/- and gadd45b-/- colony forming units granulocyte/macrophage progenitors displayed prolonged proliferation capacity compared to wt controls upon re-plating in methylcellulose supplemented with interleukin-3. The recovery of the BM myeloid compartment following 5-Fluorouracil-induced myelo-ablation was much slower in gadd45a-/- and gadd45b-/- mice compared to wt controls. Furthermore, the response of myeloid cells to inflammatory stress, inflicted via intraperitoneal administration of sodium caseinate was impaired in gadd45a-/- and gadd45b-/- mice compared to age-matched wt mice, as indicated by lower percentage of Gr-1-positive cells in the BM and lower number of myeloid cells in peritoneal exudates. Overall, these data indicate that both gadd45a and gadd45b play a role in modulating physiological stress responses of myeloid cells to acute stimulation with differentiating cytokines, myelo-ablation and inflammation. These findings should aid in understanding the response of normal and malignant hematopoietic cells to physiological and chemical stressors including anticancer agents.

Physiological pathway of differentiation of hematopoietic stem cell population into mural cells

Endothelial cells (ECs), which are a major component of blood vessels, have been reported to develop in adulthood from hematopoietic cell populations, especially those of the monocyte lineage. Here we show that mural cells (MCs), another component of blood vessels, develop physiologically during embryogenesis from a hematopoietic stem cell (HSC) population, based on the in vitro culture of HSCs and histological examination of acute myeloid leukemia 1 mutant embryos, which lack HSCs. As in the embryo, HSCs in adult bone marrow differentiate into CD45⁺CD11b⁺ cells before differentiating into MCs. Moreover, CD45⁺CD11b⁺ cells are composed of two populations, CD11b^high and CD11b^low cells, both of which can differentiate into MCs as well as ECs. Interestingly, in a murine ischemia model, MCs and ECs derived from the CD11b^low population had a long-term potential to contribute to the formation of newly developed blood vessels in vivo compared with the CD11^high population, which could not. Moreover, injection of the CD11b^high population induced leaky blood vessels, but the CD11b^low population did not. With respect to the permeability of vessels, we found that angiopoietin 1, which is a ligand for Tie2 receptor tyrosine kinase expressed on ECs and is suggested to induce cell adhesion between ECs and MCs, is produced by the CD11b^low population and plays a critical role in the formation of nonleaky vessels. These observations suggested that the CD11^low cell population serves as a good source of cells for in vivo blood vessel regeneration.

Bone-marrow haematopoietic-stem-cell niches

Adult stem cells hold many promises for future clinical applications and regenerative medicine. The haematopoietic stem cell (HSC) is the best-characterized somatic stem cell so far, but in vitro expansion has been unsuccessful, limiting the future therapeutic potential of these cells. Here we review recent progress in characterizing the composition of the HSC bone-marrow microenvironment, known as the HSC niche. During homeostasis, HSCs, and therefore putative bone-marrow HSC niches, are located near bone surfaces or are associated with the sinusoidal endothelium. The molecular crosstalk between HSCs and the cellular constituents of these niches is thought to control the balance between HSC self-renewal and differentiation, indicating that future successful expansion of HSCs for therapeutic use will require three-dimensional reconstruction of a stem-cell-niche unit.

Enforced expression of cytosolic 5'-nucleotidase I confers resistance to nucleoside analogues in vitro but systemic chemotherapy toxicity precludes in vivo selection

PURPOSE

Retroviral transfer of cDNA sequences that confer drug resistance can be used to protect against chemotherapy-induced hematopoietic toxicity and for the selective expansion of gene-modified cells. To successfully expand genetically engineered cells in vivo, an appropriate balance must be achieved between systemic toxicity induced by the selecting agent and the expansion of modified cells.

METHOD

In this study, we investigate retroviral transfer of cytosolic 5'-nucleotidase I (cN-I) for protection and selection of gene-modified cells when treated with 2-chloro-2'-deoxyadenosine (2-CdA) and 5-fluorouracil (5-FU) alone and in combination. We also attempt to design a treatment strategy for the potential in vivo selection of cN-I-modified cells by administering 5-FU to mice prior to 2-CdA treatment.

RESULTS

Our results show that cN-I can be transferred by recombinant retroviruses, and that enforced expression of cN-I protects murine fibroblast and hematopoietic progenitor cells from the cytotoxic effects of 2-CdA and/or 5-FU. Furthermore, we show that the combined administration of 5-FU and 2-CdA potentiates hematopoietic stem cell toxicity. However, the treatment also results in severe myelosuppression.

CONCLUSION

These results show that while cN-I provides both protective and selective benefits to gene-modified cells in vitro, selection requires a treatment strategy that is likely too toxic to consider cN-I as an in vivo selectable marker.

Defining the oncogenic function of the TEL/AML1 (ETV6/RUNX1) fusion protein in a mouse model

The t(12;21) translocation, generating the TEL/AML1 fusion protein, is the most common genetic lesion in childhood cancer. Using a bone marrow transplantation model, we demonstrate that TEL/AML1 expression impinges on normal hematopoietic differentiation, leading to the in vivo accumulation and persistence of an early progenitor compartment with a Sca1(+)/Kit(hi)/CD11b(+) phenotype and an increased self-renewal capacity, as documented by replating assays in vitro. Differentiation of these cells is not blocked, but the frequency of mature blood cells arising from TEL/AML1-transduced progenitors is low. Impaired differentiation is prominently observed in the pro-B-cell compartment, resulting in an proportional increase in early progenitors in vivo, consistent with the t(12;21) ALL phenotype. Despite the accumulation of both multipotent and B-cell progenitors in vivo, no leukemia induction was observed during an observation period of over 1 year. These results are consistent with findings in twins with concordant ALL, showing that TEL/AML1 generates a preleukemic clone in utero that persists for several years in a clinically covert fashion. Furthermore, our studies showed that the pointed domain of TEL/AML1, which recruits transcriptional repressors and directs oligomerization with either TEL/AML1 or wild-type TEL, was essential for the observed differentiation impairment and could not be replaced with another oligomerization domain.

Simple and Efficient Isolation of Hematopoietic Stem Cells from H2K-zFP Transgenic Mice

We have generated a transgenic mouse line that allows for simple and highly efficient enrichment for mouse hematopoietic stem cells (HSCs). The transgene expresses a green fluorescent protein variant (zFP) under the control of H2Kb promoter/enhancer element. Despite the broad zFP expression, transgenic HSCs express exceptionally high levels of zFP, allowing prospective isolation of a population highly enriched in HSCs by sorting the 0.2% of the brightest green cells from the enriched bone marrow of H2K-zFP mice. Up to 90% of zFP(bright) cells are also c-kit(high), Sca-1(high), Lin(neg), Flk-2(neg), which is a bona fide phenotype for long-term HSCs. Double-sorted zFP(bright) HSCs were capable of long-term multilineage reconstitution at a limiting dilution dose of approximately 12 cells, which is comparable to that of highly purified HSCs obtained by conventional multicolor flow cytometry. Thus, the H2K-zFP transgenic mice provide a straightforward and easy setup for the simple and highly efficient enrichment for genetically labeled HSCs without using fluorescence-conjugated monoclonal antibodies. This approach will greatly facilitate gene transfer, including short interfering RNA for gene knockdown, into HSCs and, consequently, into all other hematopoietic lineages.

SLAM family markers are conserved among hematopoietic stem cells from old and reconstituted mice and markedly increase their purity

Recent advances have increased the purity of hematopoietic stem cells (HSCs) isolated from young mouse bone marrow. However, little attention has been paid to the purity of HSCs from other contexts. Although Thy-1 low Sca-1+ Lineage- c-kit+ cells from young bone marrow are highly enriched for HSCs (1 in 5 cells gives long-term multilineage reconstitution after transplantation into irradiated mice), the same population from old, reconstituted, or cytokine-mobilized mice engrafts much less efficiently (1 in 78 to 1 in 185 cells gives long-term multilineage reconstitution). To test whether we could increase the purity of HSCs isolated from these contexts, we examined the SLAM family markers CD150 and CD48. All detectable HSCs from old, reconstituted, and cyclophosphamide/G-CSF-mobilized mice were CD150+ CD48-, just as in normal young bone marrow. Thy-1 low Sca-1+ Lineage- c-kit+ cells from old, reconstituted, or mobilized mice included mainly CD48+ and/or CD150- cells that lacked reconstituting ability. CD150+ CD48- Sca-1+ Lineage- c-kit+ cells from old, reconstituted, or mobilized mice were much more highly enriched for HSCs, with 1 in 3 to 1 in 7 cells giving long-term multilineage reconstitution. SLAM family receptor expression is conserved among HSCs from diverse contexts, and HSCs from old, reconstituted, and mobilized mice engraft relatively efficiently after transplantation when contaminating cells are eliminated.