TPO signal for stem cell genomic integrity

In this issue of Blood, de Laval et al report on a novel mechanism by which hematopoietic stem cells (HSCs) harboring DNA damage are rescued by thrombopoietin (TPO)-mediated DNA repair.1 It has been recently demonstrated that HSCs use the error-prone nonhomologous end-joining (NHEJ) pathway of DNA repair to fix DNA breaks. Maintenance of genomic integrity is crucial for HSC function. Finding the players involved in HSC DNA repair will provide a better understanding of hematopoietic homeostasis, HSC aging, and leukemogenesis.

Thrombopoietin from beginning to end

In the two decades since its cloning, thrombopoietin (TPO) has emerged not only as a critical haematopoietic cytokine, but also serves as a great example of bench-to-bedside research. Thrombopoietin, produced by the liver, is the primary regulator of megakaryocyte progenitor expansion and differentiation. Additionally, as TPO is vital for the maintenance of haematopoietic stem cells, it can truly be described as a pan-haematopoietic cytokine. Since recombinant TPO became available, the molecular mechanisms of TPO function have been the subject of extensive research. Via its receptor, c-Mpl (also termed MPL), TPO activates a wide array of downstream signalling pathways, promoting cellular survival and proliferation. Due to its central, non-redundant role in haematopoiesis, alterations of both the hormone and its receptor contribute to human disease; congenital and acquired states of thrombocytosis and thrombocytopenia and aplastic anaemia as a result from dysregulated TPO expression or functional alterations of c-Mpl. With TPO mimetics now in clinical use, the story of this haematopoietic cytokine represents a great success for biomedical research.

Thrombopoietin promotes NHEJ DNA repair in hematopoietic stem cells through specific activation of Erk and NF-κB pathways and their target, IEX-1

Loss of hematopoietic stem cell (HSC) function and increased risk of developing hematopoietic malignancies are severe and concerning complications of anticancer radiotherapy and chemotherapy. We have previously shown that thrombopoietin (TPO), a critical HSC regulator, ensures HSC chromosomal integrity and function in response to γ-irradiation by regulating their DNA-damage response. TPO directly affects the double-strand break (DSB) repair machinery through increased DNA-protein kinase (DNA-PK) phosphorylation and nonhomologous end-joining (NHEJ) repair efficiency and fidelity. This effect is not shared by other HSC growth factors, suggesting that TPO triggers a specific signal in HSCs facilitating DNA-PK activation upon DNA damage. The discovery of these unique signaling pathways will provide a means of enhancing TPO-desirable effects on HSCs and improving the safety of anticancer DNA agents. We show here that TPO specifically triggers Erk and nuclear factor κB (NF-κB) pathways in mouse hematopoietic stem and progenitor cells (HSPCs). Both of these pathways are required for a TPO-mediated increase in DSB repair. They cooperate to induce and activate the early stress-response gene, Iex-1 (ier3), upon DNA damage. Iex-1 forms a complex with pERK and the catalytic subunit of DNA-PK, which is necessary and sufficient to promote TPO-increased DNA-PK activation and NHEJ DSB repair in both mouse and human HSPCs.

Detection of cytokines in supernatant from hematopoietic stem/progenitor cells co-cultured with mesenchymal stem cells and endothelial progenitor cells

This study aimed to investigate the significance of cytokine expression in supernatant from hematopoietic stem/progenitor cells (HSCs/HPCs) co-cultured with mesenchymal stem cells (MSCs) or endothelial progenitor cells (EPCs). Mononuclear cells (MNCs) were isolated from normal human umbilical cord blood and then cultured solely or co-cultured with MSCs or EPCs. Changes in the number of MNCs and HSCs/HPCs were observed, and MNC proliferation was tested by carboxyfluorescein diacetate succinimidyl ester. The cultured supernatants of the treated MSCs and EPCs were collected at 24 h after co-culture and used to determine the concentrations of IL-3, IL-6, stem cell factor (SCF), TPO, Flt3l, and VEGF. The total number and proliferation of MNCs increased significantly when co-cultured with MSCs or EPCs than when cultured alone, particularly when MNCs were co-cultured with EPCs. The differences in IL-3 and Flt3l concentrations between groups were not significant. However, IL-6 in the MSC group was significantly higher than that in the two other groups. The SCF and TPO concentrations were highly expressed in the EPC group. The VEGF concentrations in the MSC group and the EPC group were higher than those in the control group. These results indicated that MSCs and EPCs possibly favor the proliferation of MNCs and HSCs/HPCs. IL-6 and VEGF may be related to hematopoietic reconstitution and homing ability of HSCs/HPCs. TPO may have a specific relationship with the promotion of HSCs/HPCs differentiation.

The TPO/c-MPL pathway in the bone marrow may protect leukemia cells from chemotherapy in AML Patients

Accumulating evidence indicates that the interaction of human LSCs (leukemic stem cells) with the hematopoietic microenvironment, mediated by the thrombopoietin (TPO)/c-MPL pathway, may be an underlying mechanism for resistance to cell cycle-dependent cytotoxic chemotherapy. However, the role of TPO/c-MPL signaling in AML (acute myelogenous leukemia) chemotherapy resistance hasn't been fully understood. The c-MPL and TPO levels in different AML samples were measured by flow cytometry and ELISA. We also assessed the TPO levels in the osteoblasts derived from bone mesenchymal stem cells (BMSCs). The survival rate of an AML cell line that had been co-cultured with different BMSC-derived osteoblasts was measured to determine the IC50 of an AML chemotherapy drug daunorubicin (DNR). The levels of TPO/c-MPL in the initial and relapse AML patients were significantly higher than that in the control (P < 0.05). The osteoblasts derived from AML patients' BMSCs secreted more TPO than the osteoblasts derived from normal control BMSCs (P < 0.05). A strong positive correlation between the TPO level and c-MPL expression was found in the bone marrow mononuclear cells of the relapse AML patients. More importantly, the IC50 of DNR in the HEL + AML-derived osteoblasts was the highest among all co-culture systems. High level of TPO/c-MPL signaling may protect LSCs from chemotherapy in AML. The effects of inhibition of the TPO/c-MPL pathway on enhancing the chemotherapy sensitivity of AML cells, and on their downstream effector molecules that direct the interactions between patient-derived blasts and leukemia repopulating cells need to be further studied.

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.

Human hemato-lymphoid system mice: current use and future potential for medicine

To directly study complex human hemato-lymphoid system physiology and respective system-associated diseases in vivo, human-to-mouse xenotransplantation models for human blood and blood-forming cells and organs have been developed over the past three decades. We here review the fundamental requirements and the remarkable progress made over the past few years in improving these systems, the current major achievements reached by use of these models, and the future challenges to more closely model and study human health and disease and to achieve predictive preclinical testing of both prevention measures and potential new therapies.

Signaling profiling at the single-cell level identifies a distinct signaling signature in murine hematopoietic stem cells

Hematopoietic stem cell (HSC) function is tightly regulated by cytokine signaling. Although phospho-flow cytometry allows us to study signaling in defined populations of cells, there has been tremendous hurdle to carry out this study in rare HSCs due to unrecoverable critical HSC markers, low HSC number, and poor cell recovery rate. Here, we overcame these difficulties and developed a "HSC phospho-flow" method to analyze cytokine signaling in murine HSCs at the single-cell level and compare HSC signaling profile to that of multipotent progenitors (MPPs), a cell type immediately downstream of HSCs, and commonly used Lin(-) cKit(+) cells (LK cells, enriched for myeloid progenitors). We chose to study signaling evoked from three representative cytokines, stem cell factor (SCF) and thrombopoietin (TPO) that are essential for HSC function and granulocyte macrophage-colony-stimulating factor (GM-CSF) that is dispensable for HSCs. HSCs display a distinct TPO and GM-CSF signaling signature from MPPs and LK cells, which highly correlates with receptor surface expression. In contrast, although majority of LK cells express lower levels of cKit than HSCs and MPPs, SCF-evoked ERK1/2 activation in LK cells shows a significantly increased magnitude for a prolonged period. These results suggest that specific cellular context plays a more important role than receptor surface expression in SCF signaling. Our study of HSC signaling at the homeostasis stage paves the way to investigate signaling changes in HSCs under conditions of stress, aging, and hematopoietic diseases.

Thrombopoietin/MPL participates in initiating and maintaining RUNX1-ETO acute myeloid leukemia via PI3K/AKT signaling

Oncogenic mutations in components of cytokine signaling pathways elicit ligand-independent activation of downstream signaling, enhancing proliferation and survival in acute myeloid leukemia (AML). The myeloproliferative leukemia virus oncogene, MPL, a homodimeric receptor activated by thrombopoietin (THPO), is mutated in myeloproliferative disorders but rarely in AML. Here we show that wild-type MPL expression is increased in a fraction of human AML samples expressing RUNX1-ETO, a fusion protein created by chromosome translocation t(8;21), and that up-regulation of Mpl expression in mice induces AML when coexpressed with RUNX1-ETO. The leukemic cells are sensitive to THPO, activating survival and proliferative responses. Mpl expression is not regulated by RUNX1-ETO in mouse hematopoietic progenitors or leukemic cells. Moreover, we find that activation of PI3K/AKT but not ERK/MEK pathway is a critical mediator of the MPL-directed antiapoptotic function in leukemic cells. Hence, this study provides evidence that up-regulation of wild-type MPL levels promotes leukemia development and maintenance through activation of the PI3K/AKT axis, and suggests that inhibitors of this axis could be effective for treatment of MPL-positive AML.

Eltrombopag, a thrombopoietin receptor agonist, enhances human umbilical cord blood hematopoietic stem/primitive progenitor cell expansion and promotes multi-lineage hematopoiesis

Umbilical cord blood (UCB) transplantation has emerged as a promising therapy, but it is challenged by scarcity of stem cells. Eltrombopag is a non-peptide, thrombopoietin (TPO) receptor agonist, which selectively activates c-Mpl in humans and chimpanzees. We investigated eltrombopag's effects on human UCB hematopoietic stem cell (HSC) and hematopoietic progenitor cell (HPC) expansion, and its effects on hematopoiesis in vivo. Eltrombopag selectively augmented the expansion of human CD45+, CD34+, and CD41+ cells in bone marrow compartment without effects on mouse bone marrow cells in the NOD/SCID mice xenotransplant model. Consequently, eltrombopag increased peripheral human platelets and white blood cells. We further examined effects in the STAT and AKT signaling pathways in serum-free cultures. Eltrombopag expanded human CD34+ CD38-, CD34+, and CD41+ cells. Both eltrombopag and recombinant human TPO (rhTPO) induced phosphorylation of STAT5 of CD34+ CD41-, CD34- CD41+, and CD34- CD41- cells. rhTPO preferentially induced pSTAT3, pAKT, and more pSTAT5 in CD34- C41+ cells, while eltrombopag had no effects on pSTAT3. In conclusion, eltrombopag enhanced expansion of HSCs/HPCs of human UCB in vivo and in vitro, and promoted multi-lineage hematopoiesis through the expansion of bone marrow HSCs/HPCs of human UCB in vivo. Eltrombopag differed somewhat from rhTPO in the signal transduction pathways by favoring earlier HSC/HPC populations.

The thrombopoietin/MPL/Bcl-xL pathway is essential for survival and self-renewal in human preleukemia induced by AML1-ETO

AML1-ETO (AE) is a fusion product of translocation (8;21) that accounts for 40% of M2 type acute myeloid leukemia (AML). In addition to its role in promoting preleukemic hematopoietic cell self-renewal, AE represses DNA repair genes, which leads to DNA damage and increased mutation frequency. Although this latter function may promote leukemogenesis, concurrent p53 activation also leads to an increased baseline apoptotic rate. It is unclear how AE expression is able to counterbalance this intrinsic apoptotic conditioning by p53 to promote survival and self-renewal. In this report, we show that Bcl-xL is up-regulated in AE cells and plays an essential role in their survival and self-renewal. Further investigation revealed that Bcl-xL expression is regulated by thrombopoietin (THPO)/MPL-signaling induced by AE expression. THPO/MPL-signaling also controls cell cycle reentry and mediates AE-induced self-renewal. Analysis of primary AML patient samples revealed a correlation between MPL and Bcl-xL expression specifically in t(8;21) blasts. Taken together, we propose that survival signaling through Bcl-xL is a critical and intrinsic component of a broader self-renewal signaling pathway downstream of AML1-ETO-induced MPL.

The thrombopoietin/MPL pathway in hematopoiesis and leukemogenesis

Hematopoietic stem cells (HSC) comprise a small percentage of total hematopoietic cells. Their ability to self-renewal is key to the continuous replenishment of the hematopoietic system with newly formed functional blood cell types while maintaining their multipotential capacity. Understanding the extrinsic signals that are essential to HSC maintenance will provide insights into the regulation of hematopoiesis at its most primitive stage, and with the knowledge applied, will potentially lead to improved clinical transplantation outcomes. In this review, we will summarize the current understanding of the role of the thrombopoietin/MPL signaling pathway in HSC maintenance during adult and fetal hematopoiesis. We will also speculate on the downstream key players in the pathway based on published data, and summarize the role of this pathway in leukemia.

Thrombopoietin and hematopoietic stem cells

Thrombopoietin (TPO) is the cytokine that is chiefly responsible for megakaryocyte production but increasingly attention has turned to its role in maintaining hematopoietic stem cells (HSCs). HSCs are required to initiate the production of all mature hematopoietic cells, but this differentiation needs to be balanced against self-renewal and quiescence to maintain the stem cell pool throughout life. TPO has been shown to support HSC quiescence during adult hematopoiesis, with the loss of TPO signaling associated with bone marrow failure and thrombocytopenia. Recent studies have shown that constitutive activation mutations in Mpl contribute to myeloproliferative disease. In this review, we will discuss TPO signaling pathways, regulation of TPO levels and the role of TPO in normal hematopoiesis and during myeloproliferative disease.

Current status of thrombopoietic agents

Two different thrombopoietic drugs, romiplostim and eltrombopag, have completed Phase III trials in patients with immune-mediated thrombocytopenia and were observed to successfully improve platelet counts. This review will mostly focus on these two agents. We shall review their preclinical development as well as clinical trial results in immune-mediated thrombocytopenia. Also, we shall discuss their potential clinical use in thrombocytopenias associated with other medical conditions where reduction in platelet production is an integral part of disease manifestations.

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.

Human thrombopoietin knockin mice efficiently support human hematopoiesis in vivo

Hematopoietic stem cells (HSCs) both self-renew and give rise to all blood cells for the lifetime of an individual. Xenogeneic mouse models are broadly used to study human hematopoietic stem and progenitor cell biology in vivo. However, maintenance, differentiation, and function of human hematopoietic cells are suboptimal in these hosts. Thrombopoietin (TPO) has been demonstrated as a crucial cytokine supporting maintenance and self-renewal of HSCs. We generated RAG2(-/-)γ(c)(-/-) mice in which we replaced the gene encoding mouse TPO by its human homolog. Homozygous humanization of TPO led to increased levels of human engraftment in the bone marrow of the hosts, and multilineage differentiation of hematopoietic cells was improved, with an increased ratio of myelomonocytic verus lymphoid lineages. Moreover, maintenance of human stem and progenitor cells was improved, as demonstrated by serial transplantation. Therefore, RAG2(-/-)γ(c)(-/-) TPO-humanized mice represent a useful model to study human hematopoiesis in vivo.

Megakaryopoiesis

The process of megakaryopoiesis and platelet production is complex, with the potential for regulation at multiple stages. Megakaryocytes are derived from the hematopoietic stem cell through successive lineage commitment steps, and they undergo a unique maturation process that includes polyploidization, development of an extensive internal demarcation membrane system, and finally formation of pro-platelet processes. Platelets are shed from these processes into vascular sinusoids within the bone marrow. Megakaryocyte differentiation is regulated both positively and negatively by transcription factors and cytokine signaling. Thrombopoietin (TPO) is the most important hematopoietic cytokine for platelet production. Clinically, acquired and inherited mutations affecting megakaryocytic transcription factors and thrombopoietin signaling have been identified in disorders of thrombocytopenia and thrombocytosis.

Selective enhancement of donor hematopoietic cell engraftment by the CXCR4 antagonist AMD3100 in a mouse transplantation model

The interaction between stromal cell-derived factor-1 (SDF-1) with CXCR4 chemokine receptors plays an important role in hematopoiesis following hematopoietic stem cell transplantation. We examined the efficacy of post transplant administration of a specific CXCR4 antagonist (AMD3100) in improving animal survival and in enhancing donor hematopoietic cell engraftment using a congeneic mouse transplantation model. AMD3100 was administered subcutaneously at 5 mg/kg body weight 3 times a week beginning at day +2 post-transplant. Post-transplant administration of AMD3100 significantly improves animal survival. AMD3100 reduces pro-inflammatory cytokine/chemokine production. Furthermore, post transplant administration of AMD3100 selectively enhances donor cell engraftment and promotes recovery of all donor cell lineages (myeloid cells, T and B lymphocytes, erythrocytes and platelets). This enhancement results from a combined effect of increased marrow niche availability and greater cell division induced by AMD3100. Our studies shed new lights into the biological roles of SDF-1/CXCR4 interaction in hematopoietic stem cell engraftment following transplantation and in transplant-related mortality. Our results indicate that AMD3100 provides a novel approach for enhancing hematological recovery following transplantation, and will likely benefit patients undergoing transplantation.

Thrombopoietin in normal and neoplastic stem cell development

It has been known for sometime that thrombopoietin acts on megakaryocytic progenitor cells to stimulate platelet production. It has recently been discovered that it also stimulates the self-renewal and expansion of normal murine and human haematopoietic stem cells (HSCs) by acting on its cognate receptor, the product of the myeloproliferative leukaemia (c-MPL) proto-oncogene. The c-MPL receptor may also play an important role in the development of human myeloproliferative disorders, essential thrombocythemia, myelofibrosis and polycythemia vera, cooperating with the dysregulated Janus kinase JAK2V(617)F.

Mimicking the haematopoietic niche microenvironment provides a novel strategy for expansion of haematopoietic and megakaryocyte-progenitor cells from cord blood

Severe neutropenia and protracted thrombocytopenia remain serious clinical problems following cord blood transplantation (CBT) due to the paucity of stem and progenitor cells in the grafts. Administration of ex-vivo expanded megakaryocyte progenitor cells may facilitate platelet production. We propose a novel strategy to expand these rare cells ex-vivo, from a small portion of the cord blood (CB) unit, using fibronectin (FN), a major component of hematopoietic niches, combined with cytokines, including thrombopoietin and the hematopoietic stress-associated acetylcholinesterase readthrough peptide (ARP). Application of multiple gates and high definition flow cytometry enabled clear resolution of expanded hematopoietic stem/precursor cells (HSPC) and megakaryocyte progenitors (Mk-p) and their early subsets while eliminating positively stained non-relevant cells. FN increased viability, expansion of all CD34(+) HSPC populations and Mk-p. The combination of FN + thrombopoietin + ARP maintained and expanded very early myeloid and thrombopoietic precursors, increased the proliferation of megakaryocyte, granulocyte-macrophage and multilineage colony-forming progenitors and supported Mk maturation as measured by ploidy and glycoprotein IIb/IIIa expression by quantiative reverse transcription polymerase chain reaction. This approach, which involves expanding HSPC and Mk precursors from a small portion of the CB unit, without sacrificing the coveted stem cells, may lead to improved cell therapy modalities to facilitate earlier myelopoiesis and platelet production post-CBT.

Increased differentiation of dermal mast cells in mice lacking the Mpl gene

Thrombopoietin interactions with its receptor, Mpl, play an important role in the regulation of hematopoietic stem/progenitor cell proliferation and differentiation. In this study, we report that the mast cell restricted progenitor cells (MCP) and the mast cell precursors in the bone marrow of wild-type mice express Mpl on their surface. Furthermore, targeted deletion of the Mpl gene in mice decreases the number of MCP while increasing the number of mast cell precursors present in the marrow and spleen. It also increases the number of mast cells present in the dermis, in the peritoneal cavity, and in the gut of the mice. In addition, serosal mast cells from Mpl(null) mice have a distinctive differentiation profile similar to that expressed by wild-type dermal mast cells. These results suggest that not only does ligation of thrombopoietin with the Mpl receptor exert an effect at the mast cell restricted progenitor cell level, but also plays an unexpected yet important role in mast cell maturation.

Ubiquitination and degradation of the thrombopoietin receptor c-Mpl

Regulation of growth factor and cytokine signaling is essential for maintaining physiologic numbers of circulating hematopoietic cells. Thrombopoietin (Tpo), acting through its receptor c-Mpl, is required for hematopoietic stem cell maintenance and megakaryopoiesis. Therefore, the negative regulation of Tpo signaling is critical in many aspects of hematopoiesis. In this study, we determine the mechanisms of c-Mpl degradation in the negative regulation of Tpo signaling. We found that, after Tpo stimulation, c-Mpl is degraded by both the lysosomal and proteasomal pathways and c-Mpl is rapidly ubiquitinated. Using site-directed mutagenesis, we were able to determine that c-Mpl is ubiquitinated on both of its intracellular lysine (K) residues (K(553) and K(573)). By mutating these residues to arginine, ubiquitination and degradation were significantly reduced and caused hyperproliferation in cell lines expressing these mutated receptors. Using short interfering RNA and dominant negative overexpression, we also found that c-Cbl, which is activated by Tpo, acts as an E3 ubiquitin ligase in the ubiquitination of c-Mpl. Our findings identify a previously unknown negative regulatory pathway for Tpo signaling that may significantly impact our understanding of the mechanisms affecting the growth and differentiation of hematopoietic stem cells and megakaryocytes.

Lnk controls mouse hematopoietic stem cell self-renewal and quiescence through direct interactions with JAK2

In addition to its role in megakaryocyte production, signaling initiated by thrombopoietin (TPO) activation of its receptor, myeloproliferative leukemia virus protooncogene (c-Mpl, or Mpl), controls HSC homeostasis and self-renewal. Under steady-state conditions, mice lacking the inhibitory adaptor protein Lnk harbor an expanded HSC pool with enhanced self-renewal. We found that HSCs from Lnk-/- mice have an increased quiescent fraction, decelerated cell cycle kinetics, and enhanced resistance to repeat treatments with cytoablative 5-fluorouracil in vivo compared with WT HSCs. We further provide genetic evidence demonstrating that Lnk controls HSC quiescence and self-renewal, predominantly through Mpl. Consistent with this observation, Lnk-/- HSCs displayed potentiated activation of JAK2 specifically in response to TPO. Biochemical experiments revealed that Lnk directly binds to phosphorylated tyrosine residues in JAK2 following TPO stimulation. Of note, the JAK2 V617F mutant, found at high frequencies in myeloproliferative diseases, retains the ability to bind Lnk. Therefore, we identified Lnk as a physiological negative regulator of JAK2 in stem cells and TPO/Mpl/JAK2/Lnk as a major regulatory pathway in controlling stem cell self-renewal and quiescence.

Thrombopoietin regulates c-Myb expression by modulating micro RNA 150 expression

OBJECTIVE

Mice harboring c-Myb hypomorphic mutations display enhanced thrombopoiesis because of increased numbers of megakaryocytes and their progenitors. Thrombopoietin induces these same effects, which lead us to hypothesize that the hormone acts through modulation of c-Myb expression, as c-Myb levels falls during thrombopoietin-induced megakaryocyte (MK) maturation. Micro RNAs (miRs) downregulate gene expression by binding to the 3' untranslated region (UTR) of specific messenger RNAs (mRNAs); we noted that the 3'UTR of c-Myb contains four miR-150 binding sites.

MATERIALS AND METHODS

We used quantitative reverse transcriptase polymerase chain reaction, Western blotting, and reporter gene analyses to assess the response of c-Myb to thrombopoietin stimulation and to gain of and loss of miR-150 expression.

RESULTS

We found that thrombopoietin reduced c-Myb mRNA and protein levels within 7 hours in megakaryocytes and UT7/thrombopoietin (TPO) cells. Using a reporter gene containing the c-Myb 3'UTR region, including its four miR150 binding sites, we found that expression of miR150 reduced luciferase expression to 50% of baseline at 24 hours and to 25% at 48 hours in UT7/TPO cells. Quantitative polymerase chain reaction and Western blotting also revealed that miR-150 reduced endogenous c-Myb mRNA and protein to 50% in UT7/TPO cells, and to 65% in mature megakaryocytes. Converse experiments utilizing anti-miR150 increased luciferase activity twofold over control anti-miR. Finally, TPO increased miR150 expression 1.8-fold within 24 hours and 3.4-fold within 48 hours.

CONCLUSIONS

These findings establish that miR150 downmodulates c-Myb levels, and because TPO affects miR150 expression, our results indicate that, in addition to affecting MK progenitor cell growth, TPO downmodulates c-Myb expression through induction of miR-150.

AML1/RUNX1 works as a negative regulator of c-Mpl in hematopoietic stem cells

In this study, we analyzed the roles for AML1/RUNX1 in the regulation of the c-mpl promoter. Wild-type AML1 activated the c-mpl promoter through the proximal AML-binding site in luciferase assays using 293T and HeLa cells. In accord with this result, electrophoretic mobility shift assay and chromatin immunoprecipitation assays demonstrated that AML1 bound to this site. Next, we analyzed the function of AML1 using a mutant of AML1 lacking the C terminus (AML1dC), which was originally found in a patient with myelodysplastic syndromes. AML1dC dominant-negatively suppressed transcriptional activity of wild-type AML1. However, unexpectedly, AML1dC-transduced murine c-Kit(+)Sca1(+)Lineage(-) cells expressed c-mpl mRNA and c-Mpl protein more abundantly than mock-transduced cells, which led to the enhanced thrombopoietin-mediated proliferation. Moreover, when AML1dC was induced to express during the development of hematopoietic cells from embryonic stem (ES) cells, AML1dC augmented the c-Mpl expression on hematopoietic stem/progenitor cells. Furthermore, we found that early hematopoietic cells that derived from AML1(+/-) ES cells expressed c-Mpl more intensely than those that developed from wild-type ES cells. In contrast, AML1dC hardly affected c-Mpl expression and maturation of megakaryocytes. As for the mechanism of the different roles of AML1 in the regulation of the c-mpl promoter, we found that AML1 forms a complex with a transcription repressor mSin3A on the c-mpl promoter in hematopoietic stem/progenitor cells, although it forms a complex with a transcription activator p300 on the same promoter in megakaryocytic cells. Together, these data indicate that AML1 can regulate the c-mpl promoter both positively and negatively by changing the binding partner according to cell types.

YRRL motifs in the cytoplasmic domain of the thrombopoietin receptor regulate receptor internalization and degradation

Thrombopoietin (Tpo), acting through the c-Mpl receptor, promotes the survival and proliferation of hematopoietic stem and progenitor cells and drives megakaryocyte differentiation. The proproliferation and survival signals activated by Tpo must therefore be tightly regulated to prevent uncontrolled cell growth. In this work, we determined the mechanisms that control Tpo-stimulated c-Mpl internalization and defined the processes leading to its degradation. Stimulation of BaF-Mpl cells with Tpo leads to rapid, clathrin-dependent endocytosis of the receptor. Using small interfering RNA (siRNA), we found that inhibition of adaptor protein 2 (AP2), which mediates endocytosis of transmembrane proteins, strongly attenuates Tpo-stimulated c-Mpl internalization. AP2 interacts with YXXPhi motifs and we identified 2 such motifs in c-Mpl (Y(8)RRL and Y(78)RRL) and investigated Tpo-stimulated internalization of receptors bearing point mutations at these sites. After Tpo stimulation, internalization was greatly reduced in c-Mpl Y(78)F and c-Mpl Y(8+78)F, and these cell lines also exhibited increased proliferation and increased strength and duration of Jak2, STAT5, AKT, and ERK1/2 activation in response to Tpo. We also found that the Y(8)RRL motif regulates Tpo-stimulated lysosomal degradation of c-Mpl. Our data establishes that c-Mpl cytoplasmic YRRL motifs are responsible for both Tpo-mediated internalization via interactions with AP2 and lysosomal targeting after endocytosis.

Thrombopoietin (TPO) regulates HIF-1alpha levels through generation of mitochondrial reactive oxygen species

Hypoxia inducible factor (HIF)-1 is a master transcriptional regulator mediating the cellular adaptation to hypoxia. In addition, HIF-1 is also vital for the development of hematopoietic stem cells (HSCs). In a previous study we found that thrombopoietin (TPO), an important and non-redundant cytokine for HSC maintenance and expansion, induces HIF-1alpha expression in HSCs by enhancing the stability of HIF-1alpha under normoxic conditions. However, the molecular mechanisms of these effects are not yet fully understood. In this study, we explored the mechanisms and found that TPO-induced mitochondrial reactive oxygen species (ROS) played a crucial role in stabilization of HIF-1. Both ROS scavengers and inhibitors of mitochondrial electron transport completely blocked HIF-1alpha induction by TPO in UT-7/TPO cells and in primary immature mouse bone marrow cells. We also found that TPO-induced HIF-1alpha induction was tightly coupled with glucose metabolism. Inhibition of glucose transporter or glycolytic enzyme blocked HIF-1alpha elevation of TPO. These results indicate that TPO induces HIF-1alpha expression in a manner very similar to that of hypoxia.

Historical review: megakaryopoiesis and thrombopoiesis

The study of thrombopoiesis has evolved greatly since an era when platelets were termed "the dust of the blood," only about 100 years ago. During this time megakaryocytes were identified as the origin of blood platelets; marrow-derived megakaryocytic progenitor cells were functionally defined and then purified; and the primary regulator of the process, thrombopoietin, was cloned and characterized and therapeutic thrombopoietic agents developed. During this journey we continue to learn that the physiologic mechanisms that drive proplatelet formation can be recapitulated in cell-free systems and their biochemistry evaluated; the molecular underpinnings of endomitosis are being increasingly understood; the intracellular signals sent by engagement of a large number of megakaryocyte surface receptors have been defined; and many of the transcription factors that drive megakaryocytic fate determination have been identified and experimentally manipulated. While some of these biologic processes mimic those seen in other cell types, megakaryocytes and platelets possess enough unique developmental features that we are virtually assured that continued study of thrombopoiesis will yield innumerable clinical and scientific insights for many decades to come.

Critical role of thrombopoietin in maintaining adult quiescent hematopoietic stem cells

The role of cytokines in regulation of hematopoietic stem cells (HSCs) remains poorly understood. Herein we demonstrate that thrombopoietin (THPO) and its receptor, MPL, are critically involved in postnatal steady-state HSC maintenance, reflected in a 150-fold reduction of HSCs in adult Thpo(-/-) mice. Further, whereas THPO and MPL proved not required for fetal HSC expansion, HSC expansion posttransplantation was highly MPL and THPO dependent. The distinct role of THPO in postnatal HSC maintenance is accompanied by accelerated HSC cell-cycle kinetics in Thpo(-/-) mice and reduced expression of the cyclin-dependent kinase inhibitors p57(Kip2) and p19(INK4D) as well as multiple Hox transcription factors. Although also predicted to be an HSC viability factor, BCL2 failed to rescue the HSC deficiency of Thpo(-/-) mice. Thus, THPO regulates posttransplantation HSC expansion as well as the maintenance of adult quiescent HSCs, of critical importance to avoid postnatal HSC exhaustion.

Long-term platelet production assessed in NOD/SCID mice injected with cord blood CD34+ cells, thrombopoietin-amplified in clinical grade serum-free culture

OBJECTIVE

Delayed platelet recovery post-cord blood (CB) transplantation might be due to CB characteristics: low maturity of stem cell compartment, poor production of CD34+/CD41+ cells when induced to differentiate along the megakaryocytic (MK) lineage, retention of a low ploidy in the expanded MKs. Ex vivo expansion of CB hematopoietic progenitor cells for reconstitution of different human hematopoietic lineages has already been developed in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. However, optimal conditions for MK-progenitor engraftment to reduce hemorrhaging risk still to be developed. This study assesses the hypothesis that CB-CD34+ amplification with thrombopoietin (TPO) can be applied to a portion of a CB transplant unit to stimulate recovery along MK differentiation program.

MATERIALS AND METHODS

Human CB-CD34+ cells were amplified in a serum-free, clinical grade medium with 100 ng/mL TPO alone and in addition to other cytokines (Kit ligand, interleukin-6, and Flt-3 ligand). Seven-day cultured cells were transplanted into irradiated NOD/SCID mice and engraftment, megakaryocytopoiesis, and platelet production were assessed.

RESULTS

Platelet release was successful and continuously present for at least 8 weeks in NOD/SCID mice transplanted with CB cells stimulated by TPO. Thrombocytopoiesis was more effective with transplanted TPO-amplified cells than with the cytokine cocktails.

CONCLUSION

Platelet number obtained is within the minimum level considered sufficient for hemostasis. Furthermore, amplified cells maintain their self-renewal capacity and multilineage potential differentiation. Thus, transplantation of TPO-expanded CB cells has the potential favoring both platelet recovery and human engraftment.

Thrombopoietin/MPL signaling regulates hematopoietic stem cell quiescence and interaction with the osteoblastic niche

Maintenance of hematopoietic stem cells (HSCs) depends on interaction with their niche. Here we show that the long-term (LT)-HSCs expressing the thrombopoietin (THPO) receptor, MPL, are a quiescent population in adult bone marrow (BM) and are closely associated with THPO-producing osteoblastic cells. THPO/MPL signaling upregulated beta1-integrin and cyclin-dependent kinase inhibitors in HSCs. Furthermore, inhibition and stimulation of THPO/MPL pathway by treatments with anti-MPL neutralizing antibody, AMM2, and with THPO showed reciprocal regulation of quiescence of LT-HSC. AMM2 treatment reduced the number of quiescent LT-HSCs and allowed exogenous HSC engraftment without irradiation. By contrast, exogenous THPO transiently increased quiescent HSC population and subsequently induced HSC proliferation in vivo. Altogether, these observations suggest that THPO/MPL signaling plays a critical role of LT-HSC regulation in the osteoblastic niche.

A New Role of Thrombopoietin Enhancing ex Vivo Expansion of Endothelial Precursor Cells Derived from AC133-positive Cells

We previously reported that CD31(bright) cells, which were sorted from cultured AC133(+) cells of adult peripheral blood cells, differentiated more efficiently into endothelial cells than CD31(+) cells or CD31(-) cells, suggesting that CD31(bright) cells may be endothelial precursor cells. In this study, we found that CD31(bright) cells have a strong ability to release cytokines. The mixture of vascular endothelial growth factor (VEGF), thrombopoietin (TPO), and stem cell factor stimulated ex vivo expansion of the total cell number from cultured AC133(+) cells of adult peripheral blood cells and cord blood cells, resulting in incrementation of the adhesion cells, in which endothelial nitric oxide synthase and kinase insert domain-containing receptor were positive. Moreover, the mixture of VEGF and TPO increased the CD31(bright) cell population when compared with VEGF alone or the mixture of VEGF and stem cell factor. These data suggest that TPO is an important growth factor that can promote endothelial precursor cells expansion ex vivo.

Dual role of Mpl receptor during the establishment of definitive hematopoiesis

Cytokine signaling pathways are important in promoting hematopoietic stem cell (HSC) self-renewal, proliferation and differentiation. Mpl receptor and its ligand, TPO, have been shown to play an essential role in the early steps of adult hematopoiesis. We previously demonstrated that the cytoplasmic domain of Mpl promotes hematopoietic commitment of embryonic stem cells in vitro, and postulated that Mpl could be important in the establishment of definitive hematopoiesis. To answer this question, we investigated the temporal expression of Mpl during mouse development by in situ hybridization. We found Mpl expression in the HSCs clusters emerging in the AGM region, and in the fetal liver (FL) as early as E10.5. Using Mpl(-/-) mice, the functional relevance of Mpl expression was tested by comparing the hematopoietic progenitor (HP) content, long-term hematopoietic reconstitution (LTR) abilities and HSC content of control and Mpl(-/-) embryos at different times of development. In the AGM, we observed delayed production of HSCs endowed with normal LTR but presenting a self-renewal defect. During FL development, we detected a decrease in HP and HSC potential associated with a defect in amplification and self-renewal/survival of the lin(-) AA4.1(+) Sca1(+) population of HSCs. These results underline the dual role of Mpl in the generation and expansion of HSCs during establishment of definitive hematopoiesis.

Signals emanating from the membrane proximal region of the thrombopoietin receptor (mpl) support hematopoietic stem cell self-renewal

OBJECTIVE

Studies using thrombopoietin -/- (TPO(-/-)) or TPO receptor, mpl(-/-) mice have established a critical role for TPO/mpl signaling in hematopoietic stem cell (HSC) development. In this study, we further dissected mpl signaling in both megakaryopoiesis and HSC function, using mice bearing a truncated mpl receptor lacking the distal 60 amino acids (Delta60). This deletion removes three major signaling tyrosines on the mpl cytoplasmic domain, but retains the membrane proximal Box1 and Box2 domains required for JAK2 activation.

MATERIALS AND METHODS

Competitive bone marrow transplantations (BMT) and serial BMTs were performed to study HSC function. Western blot analysis was used to study TPO-stimulated signaling pathways. BM cell cultures in the presence of TPO were used to study megakaryocyte development.

RESULTS

In agreement with prior findings, we show that Delta60 BM cells cultured in TPO generated normal numbers of megakaryocytes, but with greatly reduced ploidy. As expected from the deletion of three signaling tyrosine residues, freshly isolated Delta60 megakaryocytes showed marked reduction in all known TPO-stimulated signaling pathways tested, including signal transducers and activators of transcription (Stat) 5, Stat3, Akt, and p42/44 mitogen-activated kinase. We found that Delta60 mice displayed normal short-term (ST-HSC) activities and marginally compromised long-term (LT-HSC) stem cell activities in primary transplantation. In addition, Delta60 mice supported HSC self-renewal for at least two serial BMTs.

CONCLUSION

Our data reveal a pivotal role for an unknown signal emanating from the membrane proximal region of the mpl receptor or from JAK2 itself in maintaining stem cell activity and self-renewal, in addition to its role in megakaryocytopoiesis and thrombopoiesis.

A Novel Function of Interleukin-10 Promoting Self-Renewal of Hematopoietic Stem Cells

Self-renewal of hematopoietic stem cells (HSCs) is key to their reconstituting ability, but the factors regulating the process remain poorly understood. Here, we show that Interleukin-10 (IL-10), a pleiotropic immune modulating cytokine, can also play a role in regulating HSC self-renewal. First, a quantitative decrease of primitive hematopoietic cell populations, but not more matured cells, was observed in the bone marrows of IL-10 disrupted mice as determined by long-term in vitro cultures or in vivo competitive repopulation assays. In contrast, normal HSCs from 5-fluorouracil treated marrows cultured on the IL-10 secreting stroma displayed an enhanced repopulating activity compared with cells grown on control stroma, with ninefold higher numbers of donor-derived HSCs in the reconstituted recipient marrows. Moreover, limiting dilution transplantation assay demonstrated that exogenous addition of IL-10 in the stroma-free cultures of purified Lin- Sca-1+ c-kit+ cells caused three- to fourfold higher frequencies of HSCs in the 5-day short-term culture without indirect inhibitory effect of IL-10 on tumor necrosis factor-alpha or interferon-gamma secretion. Interestingly, primitive hematopoietic cells, including Lin- Sca-1+ c-kit+ or side population cells, expressed the surface receptor for IL-10, and microenvironmental production of IL-10 was sharply increased in the osteoblasts lining the trabecular regions of the radiation-stressed marrow but not in the steady-state marrows. These results show that IL-10 may be a ligand that can stimulate self-renewal of HSCs to promote their regeneration in addition to being a ligand for immune regulation. Disclosure of potential conflicts of interest is found at the end of this article.

Thrombopoietin promotes β1-integrin–mediated adhesion in hematopoietic cells via the small GTPase Rap1

OBJECTIVE

The interactions between cells and extracellular matrices in the bone marrow microenvironment are critical for normal hematopoiesis, controlling cell survival, proliferation, differentiation, and motility. A number of hematopoietic growth factors and cytokines can mediate these interactions by changing expression and/or activity of specific integrins, or by changing cell shape. Thrombopoietin (TPO) has previously been shown to stimulate adhesion. in certain hematopoietic cell types, although the exact mechanisms by which adhesion is promoted remain elusive.

MATERIALS AND METHODS

The role of TPO in hematopoietic cell adhesion was determined with fibronectin adhesion and binding assays, flow cytometry, and immunocytochemistry using the hematopoietic cell line UT-7/TPO and bone marrow-derived primary mouse megakaryocytes. The role of Rapl in TPO-mediated adhesion was determined using a RaplGAP overexpressing UT-7/TPO cell line, in which Rapl could not be activated.

RESULTS

We found that TPO promoted hematopoietic cell adhesion by causing cytoskeletal reorganization and not by increasing integrin expression, localization, or affinity, as previously hypothesized. Through studies using the UT-7/TPO-RaplGAP cell line, we found that TPO-mediated cell shape change occurred via activation of Rapl.

CONCLUSIONS

These data demonstrate an important role for TPO in mediating interactions in the bone marrow microenvironment and make a significant contribution to our understanding of how TPO may affect hematopoiesis.

Studies of c-Mpl function distinguish the replication of hematopoietic stem cells from the expansion of differentiating clones

Three properties define hematopoietic stem cells (HSCs): their capacity for quiescence and long survival, their ability to self-renew, and their ability to give rise to a multilineage clone of differentiating and maturing blood cells. Although it is likely that different signals regulate these events, this has been difficult to dissect on a molecular level, since HSC division, their fate decisions, and the earliest differentiation events cannot be directly visualized. Our studies of c-Mpl, the cellular receptor for the cytokine thrombopoietin, suggest that c-Mpl does not control HSC numbers, as had been previously argued, but rather facilitates the early expansion of differentiating clones. These experiments provide a strategy to distinguish the actions of HSCs from earliest progenitor cells in vivo and demonstrate that a selective growth advantage at a level distal to HSC can result in a profound effect on multilineage hematopoiesis.

Haematopoietic stem and progenitor cell-targeted therapies for thrombocytopenia

This review discusses the present outlook for new thrombocytopenia therapies that induce haematopoietic stem and progenitor cells to proliferate, differentiate and produce functional platelets. A brief overview of megakaryopoiesis and its regulation by thrombopoietin (TPO) is followed by a discussion of how early experience with recombinant TPO therapies stimulated the search for novel TPO receptor ligands. A summary is then provided of the results of Phase I clinical trials with the new small molecule and peptide TPO mimetics that are in development at present. Finally, recent developments in the ex vivo expansion of primitive haematopoietic cells and the potential enhancement of cell-based therapies by haematopoietic growth factors in vivo are briefly summarised as part of a look towards the future.

Recent advances in understanding extrinsic control of hematopoietic stem cell fate

PURPOSE OF REVIEW

Hematopoietic stem cells are responsible for generating all types of blood cells. As such they are under a high degree of regulation, both internal and external. With the identification of the hematopoietic stem cell niche, there has been increased investigation into extrinsic regulation of hematopoietic stem cells with emphasis on developmental signaling pathways. The purpose of this review is to discuss recent advances and findings in how these different pathways interact to achieve a balanced control of these stem cells.

RECENT FINDINGS

Studies indicating the importance of pathways such as Wnt, Notch, bone morphogenic protein, Sonic hedgehog and fibroblast growth factor in controlling the fate of hematopoietic stem cells are the most significant recent findings. These pathways have been implicated to affect various aspects of hematopoietic stem cells, including self-renewal, proliferation and lineage determination. Equally important are studies showing, by inactivation of various pathway components, the complexity of signal integration at the stem cell level in vivo. Additionally, some recent reports have provided evidence for direct interaction or cross-talk between different signaling pathways in this regulation.

SUMMARY

We review highlights of the recent advances made toward resolving the mechanisms of external regulation of hematopoietic stem cells. Understanding the interaction of different signaling pathways in the context of the hematopoietic stem cell niche is essential for increasing their therapeutic potential.