A functional screen to identify novel effectors of hematopoietic stem cell activity

Ectopic expression of HOXC6 blocks myeloid differentiation and predisposes to malignant transformation

Insertional mutagenesis resulting from the integration of retroviral vectors has led to the discovery of many oncogenes associated with leukemia. We investigated the role of HOXC6, identified by proximal provirus integration in a large animal hematopoietic stem cell gene therapy study, for a potential involvement in hematopoietic stem cell activity and hematopoietic cell fate decision. HOXC6 was overexpressed in the murine bone marrow transplantation model and tested in a competitive repopulation assay in comparison to the known hematopoietic stem cell expansion factor, HOXB4. We have identified HOXC6 as a factor that enhances competitive repopulation capacity in vivo and colony formation in vitro. Ectopic HOXC6 expression also induced strong myeloid differentiation and expansion of granulocyte-macrophage progenitors/common myeloid progenitors (GMPs/CMPs) in vivo, resulting in myeloid malignancies with low penetrance (3 of 17 mice), likely in collaboration with Meis1 because of a provirus integration mapped to the 3' region in the malignant clone. We characterized the molecular basis of HOXC6-induced myeloid differentiation and malignant cell transformation with complementary DNA microarray analysis. Overexpression of HOXC6 induced a gene expression signature similar to several acute myeloid leukemia subtypes when compared with normal GMPs/CMPs. These results demonstrate that HOXC6 acts as a regulator in hematopoiesis and is involved in malignant transformation.

Transcriptional regulation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are a rare cell population found in the bone marrow of adult mammals and are responsible for maintaining the entire haematopoietic system. Definitive HSCs are produced from mesoderm during embryonic development, from embryonic day 10 in the mouse. HSCs seed the foetal liver before migrating to the bone marrow around the time of birth. In the adult, HSCs are largely quiescent but have the ability to divide to self-renew and expand, or to proliferate and differentiate into any mature haematopoietic cell type. Both the specification of HSCs during development and their cellular choices once formed are tightly controlled at the level of transcription. Numerous transcriptional regulators of HSC specification, expansion, homeostasis and differentiation have been identified, primarily from analysis of mouse gene knockout experiments and transplantation assays. These include transcription factors, epigenetic modifiers and signalling pathway effectors. This chapter reviews the current knowledge of these HSC transcriptional regulators, predominantly focusing on the transcriptional regulation of mouse HSCs, although transcriptional regulation of human HSCs is also mentioned where relevant. Due to the breadth and maturity of this field, we have prioritised recently identified examples of HSC transcriptional regulators. We go on to highlight additional layers of control that regulate expression and activity of HSC transcriptional regulators and discuss how chromosomal translocations that result in fusion proteins of these HSC transcriptional regulators commonly drive leukaemias through transcriptional dysregulation.

Nfix is a novel regulator of murine hematopoietic stem and progenitor cell survival

Hematopoietic stem cells are both necessary and sufficient to sustain the complete blood system of vertebrates. Here we show that Nfix, a member of the nuclear factor I (Nfi) family of transcription factors, is highly expressed by hematopoietic stem and progenitor cells (HSPCs) of murine adult bone marrow. Although short hairpin RNA-mediated knockdown of Nfix expression in Lineage(-)Sca-1(+)c-Kit(+) HSPCs had no effect on in vitro cell growth or viability, Nfix-depleted HSPCs displayed a significant loss of colony-forming potential, as well as short- and long-term in vivo hematopoietic repopulating activity. Analysis of recipient mice at 4 to 20 days posttransplant revealed that Nfix-depleted HSPCs are established in the bone marrow, but fail to persist due to increased apoptotic cell death. Gene expression profiling of Nfix-depleted HSPCs reveals that loss of Nfix expression in HSPCs is concomitant with a decrease in the expression of multiple genes known to be important for HSPCs survival, such as Erg, Mecom, and Mpl. These data reveal that Nfix is a novel regulator of HSPCs survival posttransplantation and establish a role for Nfi genes in the regulation of this cellular compartment.

RNAi screen identifies Jarid1b as a major regulator of mouse HSC activity

Histone methylation is a dynamic and reversible process proposed to directly impact on stem cell fate. The Jumonji (JmjC) domain-containing family of demethylases comprises 27 members that target mono-, di-, and trimethylated lysine residues of histone (or nonhistone) proteins. To evaluate their role in regulation of hematopoietic stem cell (HSC) behavior, we performed an in vivo RNAi-based functional screen and demonstrated that Jarid1b and Jhdm1f play opposing roles in regulation of HSC activity. Decrease in Jarid1b levels correlated with an in vitro expansion of HSCs with preserved long-term in vivo lymphomyeloid differentiation potential. Through RNA sequencing analysis, Jarid1b knockdown was associated with increased expression levels of several HSC regulators (Hoxa7, Hoxa9, Hoxa10, Hes1, Gata2) and reduced levels of differentiation-associated genes. shRNA against Jhdmlf, in contrast, impaired hematopoietic reconstitution of bone marrow cells. Together, our studies identified Jarid1b as a negative regulator of HSC activity and Jhdmlf as a positive regulator of HSC activity.

Transcriptome analysis identifies regulators of hematopoietic stem and progenitor cells

Hematopoietic stem cells (HSCs) maintain blood homeostasis and are the functional units of bone marrow transplantation. To improve the molecular understanding of HSCs and their proximal progenitors, we performed transcriptome analysis within the context of the ImmGen Consortium data set. Gene sets that define steady-state and mobilized HSCs, as well as hematopoietic stem and progenitor cells (HSPCs), were determined. Genes involved in transcriptional regulation, including a group of putative transcriptional repressors, were identified in multipotent progenitors and HSCs. Proximal promoter analyses combined with ImmGen module analysis identified candidate regulators of HSCs. Enforced expression of one predicted regulator, Hlf, in diverse HSPC subsets led to extensive self-renewal activity ex vivo. These analyses reveal unique insights into the mechanisms that control the core properties of HSPCs.

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HSCs are transcriptionally primed for rapid activation

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A total of 322 HSC-enriched genes, including 51 transcription factors, were identified

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Ectopic Hlf expression increased ex vivo self-renewal in stem and progenitor cells

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Analysis revealed a transcription factor family hypothesized to regulate multipotency

Induction of a hemogenic program in mouse fibroblasts

Definitive hematopoiesis emerges during embryogenesis via an endothelial-to-hematopoietic transition. We attempted to induce this process in mouse fibroblasts by screening a panel of factors for hemogenic activity. We identified a combination of four transcription factors, Gata2, Gfi1b, cFos, and Etv6, that efficiently induces endothelial-like precursor cells, with the subsequent appearance of hematopoietic cells. The precursor cells express a human CD34 reporter, Sca1, and Prominin1 within a global endothelial transcription program. Emergent hematopoietic cells possess nascent hematopoietic stem cell gene-expression profiles and cell-surface phenotypes. After transgene silencing and reaggregation culture, the specified cells generate hematopoietic colonies in vitro. Thus, we show that a simple combination of transcription factors is sufficient to induce a complex, dynamic, and multistep developmental program in vitro. These findings provide insights into the specification of definitive hemogenesis and a platform for future development of patient-specific stem and progenitor cells, as well as more-differentiated blood products.

An MLL-dependent network sustains hematopoiesis

The histone methyltransferase Mixed Lineage Leukemia (MLL) is essential to maintain hematopoietic stem cells and is a leukemia protooncogene. Although clustered homeobox genes are well-characterized targets of MLL and MLL fusion oncoproteins, the range of Mll-regulated genes in normal hematopoietic cells remains unknown. Here, we identify and characterize part of the Mll-dependent transcriptional network in hematopoietic stem cells with an integrated approach by using conditional loss-of-function models, genomewide expression analyses, chromatin immunoprecipitation, and functional rescue assays. The Mll-dependent transcriptional network extends well beyond the previously appreciated Hox targets, is comprised of many characterized regulators of self-renewal, and contains target genes that are both dependent and independent of the MLL cofactor, Menin. Interestingly, PR-domain containing 16 emerged as a target gene that is uniquely effective at partially rescuing Mll-deficient hematopoietic stem and progenitor cells. This work highlights the tissue-specific nature of regulatory networks under the control of MLL/Trithorax family members and provides insight into the distinctions between the participation of MLL in normal hematopoiesis and in leukemia.

Identification of non-cell-autonomous networks from engineered feeder cells that enhance murine hematopoietic stem cell activity

In a previous gain-of-function screen, we identified 18 nuclear factors that enhance mouse hematopoietic stem cell (HSC) activity in vitro. Of these factors, the majority was believed to augment HSC function intrinsically. In the current study, we investigated the mechanisms of action of the previously identified agonists of HSC activity and tested whether human HSCs are also responsive to these factors. Our results unexpectedly revealed that the majority of the identified factors confer a competitive advantage to mouse HSCs in a non-cell-autonomous manner. Five of these factors, namely FOS, SPI1, KLF10, TFEC, and PRDM16, show robust transcriptional cross-regulation and are often associated with osteoclastogenesis. These findings define at least one novel non-cell-autonomous network in engineered niches. Surprisingly, and in contrast to their important effect on mouse HSCs, all engineered niches failed to significantly enhance the activity of human HSCs. This last finding further supports a lack of conservation in determinants that control HSC self-renewal in mouse versus human cells.

Increased learning and brain long-term potentiation in aged mice lacking DNA polymerase μ

A definitive consequence of the aging process is the progressive deterioration of higher cognitive functions. Defects in DNA repair mechanisms mostly result in accelerated aging and reduced brain function. DNA polymerase µ is a novel accessory partner for the non-homologous end-joining DNA repair pathway for double-strand breaks, and its deficiency causes reduced DNA repair. Using associative learning and long-term potentiation experiments, we demonstrate that Polµ^−/− mice, however, maintain the ability to learn at ages when wild-type mice do not. Expression and biochemical analyses suggest that brain aging is delayed in Polµ^−/− mice, being associated with a reduced error-prone DNA oxidative repair activity and a more efficient mitochondrial function. This is the first example in which the genetic ablation of a DNA-repair function results in a substantially better maintenance of learning abilities, together with fewer signs of brain aging, in old mice.

Prognostic significance of HOXB4 in de novo acute myeloid leukemia

As research into hematopoiesis advances, new factors associated with hematopoietic stem cell (HSC) activity have been discovered, and the contribution of HSC factors to hematopoiesis is now actively being investigated. Since the involvement of stem cells is considered to be important in hematological malignancies as well as normal hematopoiesis, we examined the expression of newly defined HSC factors including HOXB4, TCFEC, HMGB-1, FOS, and SPI-1 in the bone marrow (BM) from de novo acute myeloid leukemia (AML) patients. Expression levels of mRNA extracted from frozen specimens of AML patients and normal controls were measured by real-time polymerase chain reaction (PCR). Among the HSC factors, HOXB4 exhibited significantly higher expression in the BM of AML as compared with normal controls. Immunostaining for HOXB4 revealed that the HOXB4-positive cell ratios correlated well with the expression levels of mRNA for HOXB4 in AML BM. Based on the HOXB4-positive cell ratio, AML patients were divided into two groups (cases with higher ratios and lower ratios). The group with higher HOXB4-positive cell ratios had better prognoses as compared with the group with lower ratios. Multivariate analysis confirmed the HOXB4-positivity as an independent predictor of overall survival of AML patients. Lastly, mRNA expression levels for HOXB4 were inversely correlated with the expression levels of at least two genes, including ABCB1, which is known to be a multidrug-resistance gene. Our study distinguishes a subgroup of AML with higher HOXB4 expression and better prognosis, and this might be reflected by relative drug sensitivity in leukemic cells.

High-mobility group box 1: an amplifier of stem and progenitor cell activity after stroke

Stroke induces a highly complex web of pathophysiology that usually leads to serious long-term -disability. Molecules from the damage-associated molecular pattern (DAMP) family immediately increase after stroke. DAMPs are known to cause massive inflammation and brain damage. Thus, they may be targets for neuroprotection. However, emerging data now suggest that DAMPs may not always be detrimental. The high-mobility group box1 (HMGB1) protein is discussed as an example of this idea. During the acute phase after stroke, HMGB1 amplifies neuroinflammation. But during the brain remodeling phase of stroke recovery, HMGB1 can mediate beneficial plasticity and enhance stem and progenitor cell recruitment, proliferation, and differentiation within damaged brain. These emerging findings support the hypothesis that HMGB1 might be an important molecule for regulating stem and progenitor cell therapies in stroke patients.

The role of SRY-related HMG box transcription factor 4 (SOX4) in tumorigenesis and metastasis: friend or foe?

Development and progression of cancer are mediated by alterations in transcriptional networks, resulting in a disturbed balance between the activity of oncogenes and tumor suppressor genes. Transcription factors have the capacity to regulate global transcriptional profiles, and are consequently often found to be deregulated in their expression and function during tumorigenesis. Sex-determining region Y-related high-mobility-group box transcription factor 4 (SOX4) is a member of the group C subfamily of the SOX transcription factors and has a critical role during embryogenesis, where its expression is widespread and controls the development of numerous tissues. SOX4 expression is elevated in a wide variety of tumors, including leukemia, colorectal cancer, lung cancer and breast cancer, suggesting a fundamental role in the development of these malignancies. In many cancers, deregulated expression of this developmental factor has been correlated with increased cancer cell proliferation, cell survival, inhibition of apoptosis and tumor progression through the induction of an epithelial-to-mesenchymal transition and metastasis. However, in a limited subset of tumors, SOX4 has also been reported to act as a tumor suppressor. These opposing roles suggest that the outcome of SOX4 activation depends on the cellular context and the tumor origin. Indeed, SOX4 expression, transcriptional activity and target gene specificity can be controlled by signaling pathways, including the transforming growth factor-β and the WNT pathway, as well as at the post-translational level through regulation of protein stability and interaction with specific cofactors, such as TCF, syntenin-1 and p53. Here, we provide an overview of our current knowledge concerning the role of SOX4 in tumor development and progression.

An in vivo functional screen uncovers miR-150-mediated regulation of hematopoietic injury response

Hematopoietic stem and progenitor cells are often undesired targets of chemotherapies, leading to hematopoietic suppression requiring careful clinical management. Whether microRNAs control hematopoietic injury response is largely unknown. We report an in vivo gain-of-function screen and the identification of miR-150 as an inhibitor of hematopoietic recovery upon 5-fluorouracil-induced injury. Utilizing a bone marrow transplant model with a barcoded microRNA library, we screened for barcode abundance in peripheral blood of recipient mice before and after 5-fluorouracil treatment. Overexpression of screen-candidate miR-150 resulted in significantly slowed recovery rates across major blood lineages, with associated impairment of bone marrow clonogenic potential. Conversely, platelets and myeloid cells from miR-150 null marrow recovered faster after 5-fluorouracil treatment. Heterozygous knockout of c-myb, a conserved target of miR-150, partially phenocopied miR-150-forced expression. Our data highlight the role of microRNAs in controlling hematopoietic injury response and demonstrate the power of in vivo functional screens for studying microRNAs in normal tissue physiology.

HOXC4 homeoprotein efficiently expands human hematopoietic stem cells and triggers similar molecular alterations as HOXB4

BACKGROUND

Expansion of hematopoietic stem cells represents an important objective for improving cell and gene therapy protocols. Retroviral transduction of the HoxB4 homeogene in mouse and human hematopoietic stem cells and hematopoietic progenitors is known to promote the cells' expansion. A safer approach consists in transferring homeobox proteins into hematopoietic stem cells taking advantage of the natural ability of homeoproteins to cross cell membranes. Thus, HOXB4 protein transfer is operative for expanding human hematopoietic cells, but such expansion needs to be improved.

DESIGN AND METHODS

To that aim, we evaluated the effects of HOXC4, a protein encoded by a HOXB4 paralog gene, by co-culturing HOXC4-producing stromal cells with human CD34(+) hematopoietic cells. Numbers of progenitors and stem cells were assessed by in vitro cloning assays and injection into immuno-deficient mice, respectively. We also looked for activation or inhibition of target downstream gene expression.

RESULTS

We show that the HOXC4 homeoprotein expands human hematopoietic immature cells by 3 to 6 times ex vivo and significantly improves the level of in vivo engraftment. Comparative transcriptome analysis of CD34(+) cells subjected or not to HOXB4 or HOXC4 demonstrated that both homeoproteins regulate the same set of genes, some of which encode key hematopoietic factors and signaling molecules. Certain molecules identified herein are factors reported to be involved in stem cell fate or expansion in other models, such as MEF2C, EZH2, DBF4, DHX9, YPEL5 and Pumilio.

CONCLUSIONS

The present study may help to identify new HOX downstream key factors potentially involved in hematopoietic stem cell expansion or in leukemogenesis.

The CD20 homolog Ms4a8a integrates pro- and anti-inflammatory signals in novel M2-like macrophages and is expressed in parasite infection

Recently, we identified the CD20 homolog Ms4a8a as a novel molecule expressed by tumor-associated macrophages that directly enhances tumor growth. Here, we analyzed Ms4a8a(+) macrophages in M2-associated infectious pathologies. In late-stage Trypanosoma congolense and Taenia crassiceps infections, Ms4a8a expression was detected in hepatic and peritoneal macrophages respectively. Innate immunity in these infections is modulated by Toll-like receptor (TLR) signaling and TLR2/4/7 agonists strongly induced Ms4a8a expression in bone marrow derived macrophages (BMDMs) treated with M2 mediators (glucocorticoids/IL-4). LPS/dexamethasone/IL-4-induced Ms4a8a(+) BMDMs were characterized by strong expression of mRNA of mannose receptor (Mmr), arginase 1, and CD163, and by decreased iNOS expression. Coinduction of Ms4a8a by M2 mediators and TLR agonists involved the classical TLR signaling cascade via activation of MyD88/TRIF and NF-κB. Forced overexpression of Ms4a8a modulated the TLR4 response of RAW264.7 cells as shown by gene expression profiling. Upregulation of Hdc, Tcfec, and Sla was confirmed both in primary LPS/dexamethasone/IL-4-stimulated Ms4a8a(+) BMDMs and in peritoneal macrophages from late-stage Taenia crassiceps infection. In conclusion, we show that TLR signaling skews the typical alternative macrophage activation program to induce a special M2-like macrophage subset in vitro that also occurs in immunomodulatory immune reactions in vivo, a process directly involving the CD20 homolog Ms4a8a.

A novel molecular mechanism involved in multiple myeloma development revealed by targeting MafB to haematopoietic progenitors

Transgenic expression of the MafB oncogene in haematopoietic stem/progenitor cells induces plasma cell neoplasia reminiscent of human multiple myeloma and suggests DNA methylation as cause of malignant transformation.

Understanding the cellular origin of cancer can help to improve disease prevention and therapeutics. Human plasma cell neoplasias are thought to develop from either differentiated B cells or plasma cells. However, when the expression of Maf oncogenes (associated to human plasma cell neoplasias) is targeted to mouse B cells, the resulting animals fail to reproduce the human disease. Here, to explore early cellular changes that might take place in the development of plasma cell neoplasias, we engineered transgenic mice to express MafB in haematopoietic stem/progenitor cells (HS/PCs). Unexpectedly, we show that plasma cell neoplasias arise in the MafB-transgenic mice. Beyond their clinical resemblance to human disease, these neoplasias highly express genes that are known to be upregulated in human multiple myeloma. Moreover, gene expression profiling revealed that MafB-expressing HS/PCs were more similar to B cells and tumour plasma cells than to any other subset, including wild-type HS/PCs. Consistent with this, genome-scale DNA methylation profiling revealed that MafB imposes an epigenetic program in HS/PCs, and that this program is preserved in mature B cells of MafB-transgenic mice, demonstrating a novel molecular mechanism involved in tumour initiation. Our findings suggest that, mechanistically, the haematopoietic progenitor population can be the target for transformation in MafB-associated plasma cell neoplasias.

Increased HoxB4 Inhibits Apoptotic Cell Death in Pro-B Cells

HoxB4, a homeodomain-containing transcription factor, is involved in the expansion of hematopoietic stem cells and progenitor cells in vivo and in vitro, and plays a key role in regulating the balance between hematopoietic stem cell renewal and cell differentiation. However, the biological activity of HoxB4 in other cells has not been reported. In this study, we investigated the effect of overexpressed HoxB4 on cell survival under various conditions that induce death, using the Ba/F3 cell line. Analysis of phenotypical characteristics showed that HoxB4 overexpression in Ba/F3 cells reduced cell size, death, and proliferation rate. Moreover, the progression from early to late apoptotic stages was inhibited in Ba/F3 cells subjected to HoxB4 overexpression under removal of interleukin-3-mediated signal, leading to the induction of cell cycle arrest at the G2/M phase and attenuated cell death by Fas protein stimulation in vitro. Furthermore, apoptotic cell death induced by doxorubicin-treated G2/M phase cell-cycle arrest also decreased with HoxB4 overexpression in Ba/F3 cells. From these data, we suggest that HoxB4 may play an important role in the regulation of pro-B cell survival under various apoptotic death environments.

Posttranslational regulation of self-renewal capacity: insights from proteome and phosphoproteome analyses of stem cell leukemia

We recently generated 2 phenotypically similar Hoxa9+Meis1 overexpressing acute myeloid leukemias that differ by their in vivo biologic behavior. The first leukemia, named FLA2, shows a high frequency of leukemia stem cells (LSCs; 1 in 1.4 cells), whereas the second, FLB1, is more typical with a frequency of LSCs in the range of 1 per several hundred cells. To gain insights into possible mechanisms that determine LSC self-renewal, we profiled and compared the abundance of nuclear and cytoplasmic proteins and phosphoproteins from these leukemias using quantitative proteomics. These analyses revealed differences in proteins associated with stem cell fate, including a hyperactive p38 MAP kinase in FLB1 and a differentially localized Polycomb group protein Ezh2, which is mostly nuclear in FLA2 and predominantly cytoplasmic in FLB1. Together, these newly documented proteomes and phosphoproteomes represent a unique resource with more than 440 differentially expressed proteins and 11 543 unique phosphopeptides, of which 80% are novel and 7% preferentially phosphorylated in the stem cell-enriched leukemia.

The HOXB4 homeoprotein promotes the ex vivo enrichment of functional human embryonic stem cell-derived NK cells

Human embryonic stem cells (hESCs) can be induced to differentiate into blood cells using either co-culture with stromal cells or following human embryoid bodies (hEBs) formation. It is now well established that the HOXB4 homeoprotein promotes the expansion of human adult hematopoietic stem cells (HSCs) but also myeloid and lymphoid progenitors. However, the role of HOXB4 in the development of hematopoietic cells from hESCs and particularly in the generation of hESC-derived NK-progenitor cells remains elusive. Based on the ability of HOXB4 to passively enter hematopoietic cells in a system that comprises a co-culture with the MS-5/SP-HOXB4 stromal cells, we provide evidence that HOXB4 delivery promotes the enrichment of hEB-derived precursors that could differentiate into fully mature and functional NK. These hEB-derived NK cells enriched by HOXB4 were characterized according to their CMH class I receptor expression, their cytotoxic arsenal, their expression of IFNγ and CD107a after stimulation and their lytic activity. Furthermore our study provides new insights into the gene expression profile of hEB-derived cells exposed to HOXB4 and shows the emergence of CD34⁺CD45RA⁺ precursors from hEBs indicating the lymphoid specification of hESC-derived hematopoietic precursors. Altogether, our results outline the effects of HOXB4 in combination with stromal cells in the development of NK cells from hESCs and suggest the potential use of HOXB4 protein for NK-cell enrichment from pluripotent stem cells.

Concise review: Multidimensional regulation of the hematopoietic stem cell state

Hematopoietic stem cells (HSCs) are characterized by their unique function to produce all lineages of blood cells throughout life. Such tissue-specific function of HSC is attributed to their ability to execute self-renewal and multilineage differentiation. Accumulating evidence indicates that the undifferentiated state of HSC is characterized by dynamic maintenance of chromatin structures and epigenetic plasticity. Conversely, quiescence, self-renewal, and differentiation of HSCs are dictated by complex regulatory mechanisms involving specific transcription factors and microenvironmental crosstalk between stem cells and multiple compartments of niches in bone marrows. Thus, multidimensional regulatory inputs are integrated into two opposing characters of HSCs-maintenance of undifferentiated state analogous to pluripotent stem cells but execution of tissue-specific hematopoietic functions. Further studies on the interplay of such regulatory forces as "cell fate determinant" will likely shed the light on diverse spectrums of tissue-specific stem cells.

A role for GPx3 in activity of normal and leukemia stem cells

High levels of glutathione peroxidase 3 (GPx3) expression correlate with adverse prognosis in acute myeloid leukemia, and enhance activity of long-term repopulating hematopoietic stem cells in mice.

The determinants of normal and leukemic stem cell self-renewal remain poorly characterized. We report that expression of the reactive oxygen species (ROS) scavenger glutathione peroxidase 3 (GPx3) positively correlates with the frequency of leukemia stem cells (LSCs) in Hoxa9+Meis1-induced leukemias. Compared with a leukemia with a low frequency of LSCs, a leukemia with a high frequency of LSCs showed hypomethylation of the Gpx3 promoter region, and expressed high levels of Gpx3 and low levels of ROS. LSCs and normal hematopoietic stem cells (HSCs) engineered to express Gpx3 short hairpin RNA (shRNA) were much less competitive in vivo than control cells. However, progenitor cell proliferation and differentiation was not affected by Gpx3 shRNA. Consistent with this, HSCs overexpressing Gpx3 were significantly more competitive than control cells in long-term repopulation experiments, and overexpression of the self-renewal genes Prdm16 or Hoxb4 boosted Gpx3 expression. In human primary acute myeloid leukemia samples, GPX3 expression level directly correlated with adverse prognostic outcome, revealing a potential novel target for the eradication of LSCs.

Maintaining tissue homeostasis: dynamic control of somatic stem cell activity

Long-term maintenance of tissue homeostasis relies on the accurate regulation of somatic stem cell activity. Somatic stem cells have to respond to tissue damage and proliferate according to tissue requirements while avoiding overproliferation. The regulatory mechanisms involved in these responses are now being unraveled in the intestinal epithelium of Drosophila, providing new insight into strategies and mechanisms of stem cell regulation in barrier epithelia. Here, we review these studies and highlight recent findings in vertebrate epithelia that indicate significant conservation of regenerative strategies between vertebrate and fly epithelia.

Asymmetric segregation and self-renewal of hematopoietic stem and progenitor cells with endocytic Ap2a2

The stem cell–intrinsic model of self-renewal via asymmetric cell division (ACD) posits that fate determinants be partitioned unequally between daughter cells to either activate or suppress the stemness state. ACD is a purported mechanism by which hematopoietic stem cells (HSCs) self-renew, but definitive evidence for this cellular process remains open to conjecture. To address this issue, we chose 73 candidate genes that function within the cell polarity network to identify potential determinants that may concomitantly alter HSC fate while also exhibiting asymmetric segregation at cell division. Initial gene-expression profiles of polarity candidates showed high and differential expression in both HSCs and leukemia stem cells. Altered HSC fate was assessed by our established in vitro to in vivo screen on a subcohort of candidate polarity genes, which revealed 6 novel positive regulators of HSC function: Ap2a2, Gpsm2, Tmod1, Kif3a, Racgap1, and Ccnb1. Interestingly, live-cell videomicroscopy of the endocytic protein AP2A2 shows instances of asymmetric segregation during HSC/progenitor cell cytokinesis. These results contribute further evidence that ACD is functional in HSC self-renewal, suggest a role for Ap2a2 in HSC activity, and provide a unique opportunity to prospectively analyze progeny from HSC asymmetric divisions.

Tcf7 is an important regulator of the switch of self-renewal and differentiation in a multipotential hematopoietic cell line

A critical problem in biology is understanding how cells choose between self-renewal and differentiation. To generate a comprehensive view of the mechanisms controlling early hematopoietic precursor self-renewal and differentiation, we used systems-based approaches and murine EML multipotential hematopoietic precursor cells as a primary model. EML cells give rise to a mixture of self-renewing Lin-SCA+CD34+ cells and partially differentiated non-renewing Lin-SCA-CD34− cells in a cell autonomous fashion. We identified and validated the HMG box protein TCF7 as a regulator in this self-renewal/differentiation switch that operates in the absence of autocrine Wnt signaling. We found that Tcf7 is the most down-regulated transcription factor when CD34+ cells switch into CD34− cells, using RNA–Seq. We subsequently identified the target genes bound by TCF7, using ChIP–Seq. We show that TCF7 and RUNX1 (AML1) bind to each other's promoter regions and that TCF7 is necessary for the production of the short isoforms, but not the long isoforms of RUNX1, suggesting that TCF7 and the short isoforms of RUNX1 function coordinately in regulation. Tcf7 knock-down experiments and Gene Set Enrichment Analyses suggest that TCF7 plays a dual role in promoting the expression of genes characteristic of self-renewing CD34+ cells while repressing genes activated in partially differentiated CD34− state. Finally a network of up-regulated transcription factors of CD34+ cells was constructed. Factors that control hematopoietic stem cell (HSC) establishment and development, cell growth, and multipotency were identified. These studies in EML cells demonstrate fundamental cell-intrinsic properties of the switch between self-renewal and differentiation, and yield valuable insights for manipulating HSCs and other differentiating systems.

The hematopoietic system has provided a leading model for stem cell studies, and there is great interest in elucidating the mechanisms that control the decision of HSC self-renewal and differentiation. This switch is important for understanding hematopoietic diseases and manipulating HSCs for therapeutic purposes. However, because HSCs are currently unable to proliferate extensively in vitro, this severely limits the types of biochemical analyses that can be performed; and, consequently, the mechanisms that control the decision between early-stage HSC self-renewal and differentiation remain unclear. Murine bone marrow derived EML multipotential hematopoietic precursor cells are ideal for studying the switch. EML cells can grow in large culture and give rise to a mixture of self-renewing Lin-SCA+CD34+ cells and partially differentiated non-renewing Lin-SCA-CD34− cells in a cell autonomous fashion. Using RNA–Sequencing and ChIP–Sequencing, we identified and validated the HMG box protein TCF7 as a regulator in this switch and find that it operates in the absence of canonical Wnt signaling. Together with RUNX1, TCF7 regulates a network of transcription factors that characterize the CD34+ cell state. This work serves as a model for studying mechanisms of autonomous and balanced cell fate choice and is ultimately valuable for manipulating HSCs.

Myeloid translocation gene 16 is required for maintenance of haematopoietic stem cell quiescence

The t(8;21) and t(16;21) that are associated with acute myeloid leukaemia disrupt two closely related genes termed Myeloid Translocation Genes 8 (MTG8) and 16 (MTG16), respectively. Many of the transcription factors that recruit Mtg16 regulate haematopoietic stem and progenitor cell functions and are required to maintain stem cell self-renewal potential. Accordingly, we found that Mtg16-null bone marrow (BM) failed in BM transplant assays. Moreover, when removed from the animal, Mtg16-deficient stem cells continued to show defects in stem cell self-renewal assays, suggesting a requirement for Mtg16 in this process. Gene expression analysis indicated that Mtg16 was required to suppress the expression of several key cell-cycle regulators including E2F2, and chromatin immunoprecipitation assays detected Mtg16 near an E2A binding site within the first intron of E2F2. BrdU incorporation assays indicated that in the absence of Mtg16 more long-term stem cells were in the S phase, even after competitive BM transplantation where normal stem and progenitor cells are present, suggesting that Mtg16 plays a role in the maintenance of stem cell quiescence.

Expansion of functionally defined mouse hematopoietic stem and progenitor cells by a short isoform of RUNX1/AML1

Self-renewal activity is essential for the maintenance and regeneration of the hematopoietic system. The search for molecules capable of promoting self-renewal and expanding hematopoietic stem cells (HSCs) has met with limited success. Here, we show that a short isoform (AML1a) of RUNX1/AML1 has such activities. Enforced AML1a expression expanded functionally defined HSCs, with an efficiency that was at least 20 times greater than that of the control in vivo and by 18-fold within 7 days ex vivo. The ex vivo-expanded HSCs could repopulate hosts after secondary transplantations. Moreover, AML1a expression resulted in vigorous and long-term (> 10(6)-fold at 4 weeks) ex vivo expansion of progenitor cell populations capable of differentiating into multilineages. Gene expression analysis revealed that AML1a expression was associated with up-regulation of genes, including Hoxa9, Meis1, Stat1, and Ski. shRNA-mediated silencing of these genes attenuated AML1a-mediated activities. Overall, these findings establish AML1a as an isoform-specific molecule that can influence several transcriptional regulators associated with HSCs, leading to enhanced self-renewal activity and hematopoietic stem/progenitor cell expansion ex vivo and in vivo. Therefore, the abilities of AML1a may have implications for HSC transplantation and transfusion medicine, given that the effects also can be obtained by cell-penetrating AML1a protein.

Integrin-αvβ3 regulates thrombopoietin-mediated maintenance of hematopoietic stem cells

Throughout life, one's blood supply depends on sustained division of hematopoietic stem cells (HSCs) for self-renewal and differentiation. Within the bone marrow microenvironment, an adhesion-dependent or -independent niche system regulates HSC function. Here we show that a novel adhesion-dependent mechanism via integrin-β3 signaling contributes to HSC maintenance. Specific ligation of β3-integrin on HSCs using an antibody or extracellular matrix protein prevented loss of long-term repopulating (LTR) activity during ex vivo culture. The actions required activation of αvβ3-integrin "inside-out" signaling, which is dependent on thrombopoietin (TPO), an essential cytokine for activation of dormant HSCs. Subsequent "outside-in" signaling via phosphorylation of Tyr747 in the β3-subunit cytoplasmic domain was indispensable for TPO-dependent, but not stem cell factor-dependent, LTR activity in HSCs in vivo. This was accompanied with enhanced expression of Vps72, Mll1, and Runx1, 3 factors known to be critical for maintaining HSC activity. Thus, our findings demonstrate a mechanistic link between β3-integrin and TPO in HSCs, which may contribute to maintenance of LTR activity in vivo as well as during ex vivo culture.

PRDM proteins: important players in differentiation and disease

The PRDM family has recently spawned considerable interest as it has been implicated in fundamental aspects of cellular differentiation and exhibits expanding ties to human diseases. The PRDMs belong to the SET domain family of histone methyltransferases, however, enzymatic activity has been determined for only few PRDMs suggesting that they act by recruiting co-factors or, more speculatively, confer methylation of non-histone targets. Several PRDM family members are deregulated in human diseases, most prominently in hematological malignancies and solid cancers, where they can act as both tumor suppressors or drivers of oncogenic processes. The molecular mechanisms have been delineated for only few PRDMs and little is known about functional redundancy within the family. Future studies should identify target genes of PRDM proteins and the protein complexes in which PRDM proteins reside to provide a more comprehensive understanding of the biological and biochemical functions of this important protein family.

C-terminal domain of MEIS1 converts PKNOX1 (PREP1) into a HOXA9-collaborating oncoprotein

The three-amino-acid loop extension (TALE) class homeodomain proteins MEIS1 and PKNOX1 (PREP1) share the ability to interact with PBX and HOX family members and bind similar DNA sequences but appear to play opposing roles in tumor development. Elevated levels of MEIS1 accelerate development of HOX- and MLL-induced leukemias, and this pro-tumorigenic property has been associated with transcriptional activity of MEIS1. In contrast, reduction of PKNOX1 levels has been linked with cancer development despite the absence of an identifiable transactivating domain. In this report, we show that a chimeric protein generated by fusion of the MEIS1 C-terminal region encompassing the transactivating domain with the full-length PKNOX1 (PKNOX1-MC) acquired the ability to accelerate the onset of Hoxa9-induced leukemia in the mouse bone marrow transduction/transplantation model. Gene expression profiling of primary bone marrow cells transduced with Hoxa9 plus Meis1, or Hoxa9 plus Pknox1-MC revealed perturbations in overlapping functional gene subsets implicated in DNA packaging, chromosome organization, and in cell cycle regulation. Together, results presented in this report suggest that the C-terminal domain of MEIS1 confers to PKNOX1 an ectopic transactivating function that promotes leukemogenesis by regulating expression of genes involved in chromatin accessibility and cell cycle progression.

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.

The Er71 is an important regulator of hematopoietic stem cells in adult mice

The Ets transcription factor Er71 is an important regulator of endothelial and hematopoietic development during mammalian embryogenesis. However, the role of Er71 in adult hematopoiesis has remained unknown. We now first show that conditional deletion of Er71 in the hematopoietic system of adult mice results in a marked reduction (55%) in the number of hematopoietic stem cells (HSCs) that is likely due to increased cell death. Bone marrow transplantation (BMT) experiments further confirmed that Er71 is required for repopulation of HSCs. In addition, Er71(+/-) mice exhibited a slight decrease (37%) in the number of HSCs than those of Er71(+/+) mice, indicating that the function of Er71 in HSC maintenance is dependent on gene dosage. Moreover, Er71 was shown to be required for Tie2 expression, which contributes to HSC maintenance. Our results thus suggest the role of a single transcription factor in controlling HSCs through regulation of Tie2 expression in adult animals.

Cord blood-derived hematopoietic stem/progenitor cells: current challenges in engraftment, infection, and ex vivo expansion

Umbilical cord blood has served as an alternative to bone marrow for hematopoietic transplantation since the late 1980s. Numerous clinical studies have proven the efficacy of umbilical cord blood. Moreover, the possible immaturity of cells in umbilical cord blood gives more options to recipients with HLA mismatch and allows for the use of umbilical cord blood from unrelated donors. However, morbidity and mortality rates associated with hematopoietic malignancies still remain relatively high, even after cord blood transplantation. Infections and relapse are the major causes of death after cord blood transplantation in patients with hematopoietic diseases. Recently, new strategies have been introduced to improve these major problems. Establishing better protocols for simple isolation of primitive cells and ex vivo expansion will also be very important. In this short review, we discuss several recent promising findings related to the technical improvement of cord blood transplantation.

Wnt4 enhances murine hematopoietic progenitor cell expansion through a planar cell polarity-like pathway

BACKGROUND

While the role of canonical (β-catenin-mediated) Wnt signaling in hematolymphopoiesis has been studied extensively, little is known of the potential importance of non-canonical Wnt signals in hematopoietic cells. Wnt4 is one of the Wnt proteins that can elicit non-canonical pathways. We have previously shown that retroviral overexpression of Wnt4 by hematopoietic cells increased thymic cellularity as well as the frequency of early thymic progenitors and bone marrow hematopoietic progenitor cells (HPCs). However, the molecular pathways responsible for its effect in HPCs are not known.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that Wnt4 stimulation resulted in the activation of the small GTPase Rac1 as well as Jnk kinases in an HPC cell line. Jnk activity was necessary, while β-catenin was dispensable, for the Wnt4-mediated expansion of primary fetal liver HPCs in culture. Furthermore, Jnk2-deficient and Wnt4 hemizygous mice presented lower numbers of HPCs in their bone marrow, and Jnk2-deficient HPCs showed increased rates of apoptosis. Wnt4 also improved HPC activity in a competitive reconstitution model in a cell-autonomous, Jnk2-dependent manner. Lastly, we identified Fz6 as a receptor for Wnt4 in immature HPCs and showed that the absence of Wnt4 led to a decreased expression of four polarity complex genes.

CONCLUSIONS/SIGNIFICANCE

Our results establish a functional role for non-canonical Wnt signaling in hematopoiesis through a pathway involving Wnt4, Fz6, Rac1 and Jnk kinases.

Common and overlapping oncogenic pathways contribute to the evolution of acute myeloid leukemias

Fusion oncogenes in acute myeloid leukemia (AML) promote self-renewal from committed progenitors, thereby linking transformation and self-renewal pathways. Like most cancers, AML is a genetically and biologically heterogeneous disease, but it is unclear whether transformation results from common or overlapping genetic programs acting downstream of multiple mutations or by the engagement of unique genetic programs acting cooperatively downstream of individual mutations. This distinction is important, because the involvement of common programs would imply the existence of common molecular targets to treat AML, no matter which oncogenes are involved. Here we show that the ability to promote self-renewal is a generalized property of leukemia-associated oncogenes. Disparate oncogenes initiated overlapping transformation and self-renewal gene expression programs, the common elements of which were defined in established leukemic stem cells from an animal model as well as from a large cohort of patients with differing AML subtypes, where they strongly predicted pathobiological character. Notably, individual genes commonly activated in these programs could partially phenocopy the self-renewal function of leukemia-associated oncogenes in committed murine progenitors. Furthermore, they could generate AML following expression in murine bone marrow. In summary, our findings reveal the operation of common programs of self-renewal and transformation downstream of leukemia-associated oncogenes, suggesting that mechanistically common therapeutic approaches to AML are likely to be possible, regardless of the identity of the driver oncogene involved.

Genome-wide analysis of target genes regulated by HoxB4 in hematopoietic stem and progenitor cells developing from embryonic stem cells

Forced expression of the transcription factor HoxB4 has been shown to enhance the self-renewal capacity of mouse bone marrow hematopoietic stem cells (HSCs) and confer a long-term repopulating capacity to yolk sac and embryonic stem (ES) cell-derived hematopoietic precursors. The fact that ES cell-derived precursors do not repopulate bone marrow without HoxB4 underscores an important role for HoxB4 in the maturation of ES-derived hematopoietic precursors into long-term repopulating HSCs. However, the precise molecular mechanism underlying this process is barely understood. In this study, we performed a genome-wide analysis of HoxB4 using ES cell-derived hematopoietic stem/progenitor cells. The results revealed many of the genes essential for HSC development to be direct targets of HoxB4, such as Runx1, Scl/Tal1, Gata2, and Gfi1. The expression profiling also showed that HoxB4 indirectly affects the expression of several important genes, such as Lmo2, Erg, Meis1, Pbx1, Nov, AhR, and Hemgn. HoxB4 tended to activate the transcription, but the down-regulation of a significant portion of direct targets suggested its function to be context-dependent. These findings indicate that HoxB4 reprograms a set of key regulator genes to facilitate the maturation of developing HSCs into repopulating cells. Our list of HoxB4 targets also provides novel candidate regulators for HSCs.

Prdm16 is a physiologic regulator of hematopoietic stem cells

Fetal liver and adult bone marrow hematopoietic stem cells (HSCs) renew or differentiate into committed progenitors to generate all blood cells. PRDM16 is involved in human leukemic translocations and is expressed highly in some karyotypically normal acute myeloblastic leukemias. As many genes involved in leukemogenic fusions play a role in normal hematopoiesis, we analyzed the role of Prdm16 in the biology of HSCs using Prdm16-deficient mice. We show here that, within the hematopoietic system, Prdm16 is expressed very selectively in the earliest stem and progenitor compartments, and, consistent with this expression pattern, is critical for the establishment and maintenance of the HSC pool during development and after transplantation. Prdm16 deletion enhances apoptosis and cycling of HSCs. Expression analysis revealed that Prdm16 regulates a remarkable number of genes that, based on knockout models, both enhance and suppress HSC function, and affect quiescence, cell cycling, renewal, differentiation, and apoptosis to various extents. These data suggest that Prdm16 may be a critical node in a network that contains negative and positive feedback loops and integrates HSC renewal, quiescence, apoptosis, and differentiation.

The guardians of the genome (p53, TA-p73, and TA-p63) are regulators of tumor suppressor miRNAs network

The tumor suppressor p53 homologues, TA-p73, and p63 have been shown to function as tumor suppressors. However, how they function as tumor suppressors remains elusive. Here, I propose a number of tumor suppressor pathways that illustrate how the TA-p73 and p63 could function as negative regulators of invasion, metastasis, and cancer stem cells (CSCs) proliferation. Furthermore, I provide molecular insights into how TA-p73 and p63 could function as tumor suppressors. Remarkably, the guardians--p53, p73, and p63--of the genome are in control of most of the known tumor suppressor miRNAs, tumor suppressor genes, and metastasis suppressors by suppressing c-myc through miR-145/let-7/miR-34/TRIM32/PTEN/FBXW7. In particular, p53 and TA-p73/p63 appear to upregulate the expression of (1) tumor suppressor miRNAs, such as let-7, miR-34, miR-15/16a, miR-145, miR-29, miR-26, miR-30, and miR-146a; (2) tumor suppressor genes, such as PTEN, RBs, CDKN1a/b/c, and CDKN2a/b/c/d; (3) metastasis suppressors, such as Raf kinase inhibitory protein, CycG2, and DEC2, and thereby they enlarge their tumor suppressor network to inhibit tumorigenesis, invasion, angiogenesis, migration, metastasis, and CSCs proliferation.

EGF signaling regulates the proliferation of intestinal stem cells in Drosophila

Precise control of somatic stem cell proliferation is crucial to ensure maintenance of tissue homeostasis in high-turnover tissues. In Drosophila, intestinal stem cells (ISCs) are essential for homeostatic turnover of the intestinal epithelium and ensure epithelial regeneration after tissue damage. To accommodate these functions, ISC proliferation is regulated dynamically by various growth factors and stress signaling pathways. How these signals are integrated is poorly understood. Here, we show that EGF receptor signaling is required to maintain the proliferative capacity of ISCs. The EGF ligand Vein is expressed in the muscle surrounding the intestinal epithelium, providing a permissive signal for ISC proliferation. We find that the AP-1 transcription factor FOS serves as a convergence point for this signal and for the Jun N-terminal kinase (JNK) pathway, which promotes ISC proliferation in response to stress. Our results support the notion that the visceral muscle serves as a functional 'niche' for ISCs, and identify FOS as a central integrator of a niche-derived permissive signal with stress-induced instructive signals, adjusting ISC proliferation to environmental conditions.

Lentiviral gene transfer regenerates hematopoietic stem cells in a mouse model for Mpl-deficient aplastic anemia

Thpo/Mpl signaling plays an important role in the maintenance of hematopoietic stem cells (HSCs) in addition to its role in megakaryopoiesis. Patients with inactivating mutations in Mpl develop thrombocytopenia and aplastic anemia because of progressive loss of HSCs. Yet, it is unknown whether this loss of HSCs is an irreversible process. In this study, we used the Mpl knockout (Mpl(-/-)) mouse model and expressed Mpl from newly developed lentiviral vectors specifically in the physiologic Mpl target populations, namely, HSCs and megakaryocytes. After validating lineage-specific expression in vivo using lentiviral eGFP reporter vectors, we performed bone marrow transplantation of transduced Mpl(-/-) bone marrow cells into Mpl(-/-) mice. We show that restoration of Mpl expression from transcriptionally targeted vectors prevents lethal adverse reactions of ectopic Mpl expression, replenishes the HSC pool, restores stem cell properties, and corrects platelet production. In some mice, megakaryocyte counts were atypically high, accompanied by bone neo-formation and marrow fibrosis. Gene-corrected Mpl(-/-) cells had increased long-term repopulating potential, with a marked increase in lineage(-)Sca1(+)cKit(+) cells and early progenitor populations in reconstituted mice. Transcriptome analysis of lineage(-)Sca1(+)cKit(+) cells in Mpl-corrected mice showed functional adjustment of genes involved in HSC self-renewal.

Hemp, an mbt domain-containing protein, plays essential roles in hematopoietic stem cell function and skeletal formation

To clarify the molecular pathways governing hematopoietic stem cell (HSC) development, we screened a fetal liver (FL) HSC cDNA library and identified a unique gene, hematopoietic expressed mammalian polycomb (hemp), encoding a protein with a zinc-finger domain and four malignant brain tumor (mbt) repeats. To investigate its biological role, we generated mice lacking Hemp (hemp(-/-)). Hemp(-/-) mice exhibited a variety of skeletal malformations and died soon after birth. In the FL, hemp was preferentially expressed in the HSC and early progenitor cell fractions, and analyses of fetal hematopoiesis revealed that the number of FL mononuclear cells, including HSCs, was reduced markedly in hemp(-/-) embryos, especially during early development. In addition, colony-forming and competitive repopulation assays demonstrated that the proliferative and reconstitution abilities of hemp(-/-) FL HSCs were significantly impaired. Microarray analysis revealed alterations in the expression levels of several genes implicated in hematopoietic development and differentiation in hemp(-/-) FL HSCs. These results demonstrate that Hemp, an mbt-containing protein, plays essential roles in HSC function and skeletal formation. It is also hypothesized that Hemp might be involved in certain congenital diseases, such as Klippel-Feil anomaly.

Dynamic interaction networks in a hierarchically organized tissue

We have integrated gene expression profiling with database and literature mining, mechanistic modeling, and cell culture experiments to identify intercellular and intracellular networks regulating blood stem cell self-renewal.

Blood stem cell fate in vitro is regulated non-autonomously by a coupled positive–negative intercellular feedback circuit, composed of megakaryocyte-derived stimulatory growth factors (VEGF, PDGF, EGF, and serotonin) versus monocyte-derived inhibitory factors (CCL3, CCL4, CXCL10, TGFB2, and TNFSF9).

The antagonistic signals converge in a core intracellular network focused around PI3K, Raf, PLC, and Akt.

Model simulations enable functional classification of the novel endogenous ligands and signaling molecules.

Intercellular (between cell) communication networks are required to maintain homeostasis and coordinate regenerative and developmental cues in multicellular organisms. Despite the recognized importance of intercellular networks in regulating adult stem and progenitor cell fate, the specific cell populations involved, and the underlying molecular mechanisms are largely undefined. Although a limited number of studies have applied novel bioinformatic approaches to unravel intercellular signaling in other cell systems (Frankenstein et al, 2006), a comprehensive analysis of intercellular communication in a stem cell-derived, hierarchical tissue network has yet to be reported.

As a model system to explore intercellular communication networks in a hierarchically organized tissue, we cultured human umbilical cord blood (UCB)-derived stem and progenitor cells in defined, minimal cytokine-supplemented liquid culture (Madlambayan et al, 2006). To systematically explore the molecular and cellular dynamics underlying primitive progenitor growth and differentiation, gene expression profiles of primitive (lineage negative; Lin⁻) and mature (lineage positive; Lin⁺) populations were generated during phases of stem cell expansion versus depletion. Parallel phenotypic and subproteomic experiments validated that mRNA expression correlated with complex measures of proteome activity (protein secretion and cell surface expression). Using a curated list of secreted ligand–receptor interactions and published expression profiles of purified mature blood populations, we implemented a novel algorithm to reconstruct the intercellular signaling networks established between stem cells and multi-lineage progeny in vitro. By correlating differential expression patterns with stem cell growth, we predict cell populations, pathways, and secreted ligands associated with stem cell self-renewal and differentiation (Figure 3A).

We then tested the correlative predictions in a series of cell culture experiments. UCB progenitor cell cultures were supplemented with saturating amounts of 18 putative regulatory ligands, or cocultured with purified mature blood lineages (megakaryocytes, monocytes, and erythrocytes), and analyzed for effects on total cell, progenitor, and primitive progenitor growth. At the primitive progenitor level, 3/5 novel predicted stimulatory ligands (EGF, PDGFB, and VEGF) displayed significant positive effects, 5/7 predicted inhibitory factors (CCL3, CCL4, CXCL10, TNFSF9, and TGFB2) displayed negative effects, whereas only 1/5 non-correlated ligand (CXCL7) displayed an effect. Also consistent with predictions from gene expression data, megakaryocytes and monocytes were found to stimulate and inhibit primitive progenitor growth, respectively, and these effects were attributable to differential secretome profiles of stimulatory versus inhibitory ligands.

Cellular responses to external stimuli, particularly in heterogeneous and dynamic cell populations, represent complex functions of multiple cell fate decisions acting both directly and indirectly on the target (stem cell) populations. Experimentally distinguishing the mode of action of cytokines is thus a difficult task. To address this we used our previously published interactive model of hematopoiesis (Kirouac et al, 2009) to classify experimentally identified regulatory ligands into one of four distinct functional categories based on their differential effects on cell population growth. TGFB2 was classified as a proliferation inhibitor, CCL4, CXCL10, SPARC, and TNFSF9 as self-renewal inhibitors, CCL3 a proliferation stimulator, and EGF, VEGF, and PDGFB as self-renewal stimulators.

Stem and progenitor cells exposed to combinatorial extracellular signals must propagate this information through intracellular molecular networks, and respond appropriately by modifying cell fate decisions. To explore how our experimentally identified positive and negative regulatory signals are integrated at the intracellular level, we constructed a blood stem cell self-renewal signaling network through extensive literature curation and protein–protein interaction (PPI) network mapping. We find that signal transduction pathways activated by the various stimulatory and inhibitory ligands converge on a limited set of molecular control nodes, forming a core subnetwork enriched for known regulators of self-renewal (Figure 6A). To experimentally test the intracellular signaling molecules computationally predicted as regulators of stem cell self-renewal, we obtained five small molecule antagonists against the kinases Phosphatidylinositol 3-kinase (PI3K), Raf, Akt, Phospholipase C (PLC), and MEK1. Liquid cultures were supplemented with the five molecules individually, and resultant cell population outputs compared against model simulations to deconvolute the functional effects on proliferation (and survival) versus self-renewal. This analysis classifies inhibition of PI3K and Raf activity as selectively targeting self-renewal, PLC as selectively targeting survival, and Akt as selectively targeting proliferation; MEK inhibition appears non-specific for these processes.

This represents the first systematic characterization of how cell fate decisions are regulated non-autonomously through lineage-specific interactions with differentiated progeny. The complex intercellular communication networks can be approximated as an antagonistic positive–negative feedback circuit, wherein progenitor expansion is modulated by a balance of megakaryocyte-derived stimulatory factors (EGF, PDGF, VEGF, and possibly serotonin) versus monocyte-derived inhibitory factors (CCL3, CCL4, CXCL10, TGFB2, and TNFSF9). This complex milieu of endogenous regulatory signals is integrated and processed within a core intracellular signaling network, resulting in modulation of cell-level kinetic parameters (proliferation, survival, and self-renewal). We reconstruct a stem cell associated intracellular network, and identify PI3K, Raf, Akt, and PLC as functionally distinct signal integration nodes, linking extracellular and intracellular signaling. These findings lay the groundwork for novel strategies to control blood stem cell self-renewal in vitro and in vivo.

Intercellular (between cell) communication networks maintain homeostasis and coordinate regenerative and developmental cues in multicellular organisms. Despite the importance of intercellular networks in stem cell biology, their rules, structure and molecular components are poorly understood. Herein, we describe the structure and dynamics of intercellular and intracellular networks in a stem cell derived, hierarchically organized tissue using experimental and theoretical analyses of cultured human umbilical cord blood progenitors. By integrating high-throughput molecular profiling, database and literature mining, mechanistic modeling, and cell culture experiments, we show that secreted factor-mediated intercellular communication networks regulate blood stem cell fate decisions. In particular, self-renewal is modulated by a coupled positive–negative intercellular feedback circuit composed of megakaryocyte-derived stimulatory growth factors (VEGF, PDGF, EGF, and serotonin) versus monocyte-derived inhibitory factors (CCL3, CCL4, CXCL10, TGFB2, and TNFSF9). We reconstruct a stem cell intracellular network, and identify PI3K, Raf, Akt, and PLC as functionally distinct signal integration nodes, linking extracellular, and intracellular signaling. This represents the first systematic characterization of how stem cell fate decisions are regulated non-autonomously through lineage-specific interactions with differentiated progeny.

KRAS(G12V) enhances proliferation and initiates myelomonocytic differentiation in human stem/progenitor cells via intrinsic and extrinsic pathways

In human hematopoietic malignancies, RAS mutations are frequently observed. Yet, little is known about signal transduction pathways that mediate KRAS-induced phenotypes in human CD34(+) stem/progenitor cells. When cultured on bone marrow stroma, we observed that KRAS(G12V)-transduced cord blood (CB) CD34(+) cells displayed a strong proliferative advantage over control cells, which coincided with increased early cobblestone (CAFC) formation and induction of myelomonocytic differentiation. However, the KRAS(G12V)-induced proliferative advantage was transient. By week three no progenitors remained in KRAS(G12V)-transduced cultures and cells were all terminally differentiated into monocytes/macrophages. In line with these results, LTC-IC frequencies were strongly reduced. Both the ERK and p38 MAPK pathways, but not JNK, were activated by KRAS(G12V) and we observed that proliferation and CAFC formation were mediated via ERK, while differentiation was predominantly mediated via p38. Interestingly, we observed that KRAS(G12V)-induced proliferation and CAFC formation, but not differentiation, were largely mediated via secreted factors, since these phenotypes could be recapitulated by treating non-transduced cells with conditioned medium harvested from KRAS(G12V)-transduced cultures. Multiplex cytokine arrays and genome-wide gene expression profiling were performed to gain further insight into the mechanisms by which oncogenic KRAS(G12V) can contribute to the process of leukemic transformation. Thus, angiopoietin-like 6 (ANGPTL6) was identified as an important factor in the KRAS(G12V) secretome that enhanced proliferation of human CB CD34(+) cells.

An RNAi screen identifies Msi2 and Prox1 as having opposite roles in the regulation of hematopoietic stem cell activity

In this study, we describe an in vivo RNA interference functional genetics approach to evaluate the role of 20 different conserved polarity factors and fate determinants in mouse hematopoietic stem cell (HSC) activity. In total, this screen revealed three enhancers and one suppressor of HSC-derived reconstitution. Pard6a, Prkcz, and Msi2 shRNA-mediated depletion significantly impaired HSC repopulation. An in vitro promotion of differentiation was observed after the silencing of these genes, consistent with their function in regulating HSC self-renewal. Conversely, Prox1 knockdown led to in vivo accumulation of primitive and differentiated cells. HSC activity was also enhanced in vitro when Prox1 levels were experimentally reduced, identifying it as a potential antagonist of self-renewal. HSC engineered to overexpress Msi2 or Prox1 showed the reverse phenotype to those transduced with corresponding shRNA vectors. Gene expression profiling studies identified a number of known HSC and cell cycle regulators as potential downstream targets to Msi2 and Prox1.

High-mobility group B1 proteins in canine lymphoma: prognostic value of initial and sequential serum levels in treatment outcome following combination chemotherapy

Elevated high-mobility group box 1 (HMGB1) levels have been demonstrated in different human neoplasias. Information on serum HMGB1 before and during chemotherapy is lacking, as is data pertaining to its prognostic significance. The aim of this study was to characterize serum HMGB1 level in dogs with lymphoma and to assess its influence on the outcome following chemotherapy. Serum HMGB1 concentrations were measured in 16 dogs with lymphoma before treatment (W1) and on weeks 2 (W2), 6 (W6) and 12 (W12) of treatment with chemotherapy. Initial serum HMGB1 levels were significantly higher than HMGB1concentrations in control dogs and the levels in W2, W6 and W12. HMGB1-W1 concentrations were lower in dogs achieving complete remission than that in the single dog with partial remission. The ratio W12/W6 exhibited significant influence on remission duration. In these dogs with lymphoma, serum HMGB1 was elevated in comparison with that in controls. Initial serum HMGB1 level and its modulation during treatment may possess prognostic value.

Prdm16 promotes stem cell maintenance in multiple tissues, partly by regulating oxidative stress

To better understand the mechanisms that regulate stem cell identity and function we sought to identify genes that are preferentially expressed by stem cells and critical for their function in multiple tissues. Prdm16 is a transcription factor that regulates leukemogenesis1, palatogenesis2, and brown fat development3–5, but which was not known to be required for stem cell function. We demonstrate that Prdm16 is preferentially expressed by stem cells throughout the nervous and hematopoietic systems and required for their maintenance. Prdm16 deficiency led to changes in reactive oxygen species (ROS) levels, increased cell death, altered cell cycle distribution, and stem cell depletion in the hematopoietic and nervous systems. In neural stem/progenitor cells, Prdm16 bound the Hgf promoter and in the absence of Prdm16 Hgf expression declined. Addition of recombinant HGF to culture partially rescued the increase in ROS levels and the depletion of Prdm16 deficient neural stem cells. Administration of the anti-oxidant, N-acetyl-cysteine, to Prdm16 deficient mice partially rescued defects in neural stem/progenitor cell function and neural development. Prdm16 therefore promotes stem cell maintenance in multiple tissues, partly by modulating oxidative stress.

MicroRNAs enriched in hematopoietic stem cells differentially regulate long-term hematopoietic output

The production of blood cells depends on a rare hematopoietic stem-cell (HSC) population, but the molecular mechanisms underlying HSC biology remain incompletely understood. Here, we identify a subset of microRNAs (miRNAs) that is enriched in HSCs compared with other bone-marrow cells. An in vivo gain-of-function screen found that three of these miRNAs conferred a competitive advantage to engrafting hematopoietic cells, whereas other HSC miRNAs attenuated production of blood cells. Overexpression of the most advantageous miRNA, miR-125b, caused a dose-dependent myeloproliferative disorder that progressed to a lethal myeloid leukemia in mice and also enhanced hematopoietic engraftment in human immune system mice. Our study identifies an evolutionarily conserved subset of miRNAs that is expressed in HSCs and functions to modulate hematopoietic output.

SKI promotes Smad3 linker phosphorylations associated with the tumor-promoting trait of TGFbeta

The transcriptional co-regulator SKI is a potent inhibitor of TGFbeta-growth inhibitory signals. SKI binds to receptor-activated Smads in the nucleus, forming repressor complexes containing HDACs, mSin3, NCoR, and other protein partners. Alternatively, SKI binds to activated Smads in the cytoplasm, preventing their nuclear translocation. SKI is necessary for anchorage-independent growth of melanoma cells in vitro, and most important, for human melanoma xenograft growth in vivo. We recently identified a novel role of SKI in TGFbeta signaling. SKI promotes the switch of Smad3 from repressor of proliferation to activator of oncogenesis by facilitating phosphorylations in the linker domain. High levels of endogenous SKI are required by the tumor promoting trait of TGFbeta to induce expression of the plasminogen-activator inhibitor-1 (PAI-1), sustained expression of C-Myc and for aborting upregulation of p21(Waf-1). Here we discuss how SKI diversifies and amplifies its functions by associating with multiple protein partners and by promoting Smad3 linker phosphorylation(s) in response to TGFbeta signaling in melanoma cells.